Task IdentityLinkCount is incorrectly calculated if the same identityLink is deleted multiple times in the same transaction

**Describe the bug**
The `identityLinkCount` field on the `TaskEntityImpl` can be incorrectly calculated when an identitylink on the task is deleted multiple times in the same transaction, using the method `taskService.deleteUserIdentityLink(...)`.

Each time the method to delete the identityLink is executed then `identityLinkCount` on the task is reduced by one. If during the same transaction the same identityLink has already been marked for deletion, then the `identityLinkCount` is still reduced by 1 but no extra rows in the database are removed.

This can result in a negative `identityLinkCount` and causes issues when the task is completed or deleted. As a negative value for the `identityLinkCount` causes no identityLinks to be removed when the task is removed, and leaves an inconsistency in the database.

Note, this only occurs if the deletion occurs in the same transaction and if it is done in a separate transaction then the deletion is a no-op and does not remove 1 from `identityLinkCount`

This is an issue for me as I have multiple listeners which manipulate the IdentityLinks on a task. Sometimes these listeners remove the same identityLink during the same transaction and cause this error to occur.

**Expected behavior**
If the same identityLink is deleted multiple times in the same transaction, we should either get an error from the `taskService.deleteUserIdentityLink(...)` or the `identityLinkCount` should not be reduced by 1.

**Code**
Here is some code to reproduce the problem in Flowable 6.6.0

```
@FlowableTest
public class TaskIdentityLinkCountNegative {

private Logger log = LoggerFactory.getLogger(TaskIdentityLinkCountNegative.class);

private ProcessEngine processEngine;
private HistoryService historyService;
private ManagementService managementService;
private RuntimeService runtimeService;
private TaskService taskService;

@BeforeEach
void setUp(ProcessEngine processEngine) {
this.processEngine = processEngine;
this.historyService = processEngine.getHistoryService();
this.managementService = processEngine.getManagementService();
this.runtimeService = processEngine.getRuntimeService();
this.taskService = processEngine.getTaskService();
}

@Test
@Deployment(resources = {
"TaskIdentityLinkCountNegative.testUserIdentityLinkCountIsIncorrect.bpmn20.xml"
})
void testUserIdentityLinkCountIsIncorrect() {

ProcessInstance processInstance = runtimeService.startProcessInstanceByKey("my-process");
Task task = taskService.createTaskQuery().processInstanceId(processInstance.getId()).singleResult();
String taskId = task.getId();

taskService.addUserIdentityLink(taskId, "user1", IdentityLinkType.PARTICIPANT);
taskService.addUserIdentityLink(taskId, "user2", IdentityLinkType.PARTICIPANT);

managementService.executeCommand(command -> {
taskService.deleteUserIdentityLink(taskId, "user1", IdentityLinkType.PARTICIPANT);
taskService.deleteUserIdentityLink(taskId, "user1", IdentityLinkType.PARTICIPANT);
taskService.deleteUserIdentityLink(taskId, "user1", IdentityLinkType.PARTICIPANT);
return null;
});

CountingTaskEntity countingTaskEntity = (CountingTaskEntity) taskService.createTaskQuery().processInstanceId(processInstance.getId()).singleResult();

log.info("IdentityLinkCount is {}", countingTaskEntity.getIdentityLinkCount());

taskService.complete(taskId);
}
}
```

The BPMN used is

```

```

When running the test there is the following log message and database error.
In the log message we can see a negative `IdentityLinkCount`

```
12:52:26.864 [main] INFO t.TaskIdentityLinkCountNegative - IdentityLinkCount is -1
12:52:26.919 [main] ERROR o.f.c.e.i.interceptor.CommandContext - Error while closing command context
org.apache.ibatis.exceptions.PersistenceException:
### Error updating database. Cause: org.h2.jdbc.JdbcSQLException: Referential integrity constraint violation: "ACT_FK_TSKASS_TASK: PUBLIC.ACT_RU_IDENTITYLINK FOREIGN KEY(TASK_ID_) REFERENCES PUBLIC.ACT_RU_TASK(ID_) ('10')"; SQL statement:
delete from ACT_RU_TASK where ID_ = ? and REV_ = ? [23503-176]
### The error may exist in org/flowable/task/service/db/mapping/entity/Task.xml
### The error may involve org.flowable.task.service.impl.persistence.entity.TaskEntityImpl.deleteTask-Inline
### The error occurred while setting parameters
### SQL: delete from ACT_RU_TASK where ID_ = ? and REV_ = ?
### Cause: org.h2.jdbc.JdbcSQLException: Referential integrity constraint violation: "ACT_FK_TSKASS_TASK: PUBLIC.ACT_RU_IDENTITYLINK FOREIGN KEY(TASK_ID_) REFERENCES PUBLIC.ACT_RU_TASK(ID_) ('10')"; SQL statement:
delete from ACT_RU_TASK where ID_ = ? and REV_ = ? [23503-176]
at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30) ~[mybatis-3.5.5.jar:3.5.5]
at org.apache.ibatis.session.defaults.DefaultSqlSession.update(DefaultSqlSession.java:199) ~[mybatis-3.5.5.jar:3.5.5]
at org.apache.ibatis.session.defaults.DefaultSqlSession.delete(DefaultSqlSession.java:212) ~[mybatis-3.5.5.jar:3.5.5]
at org.flowable.common.engine.impl.db.DbSqlSession.flushDeleteEntities(DbSqlSession.java:641) ~[flowable-engine-common-6.6.0.jar:6.6.0]
at org.flowable.common.engine.impl.db.DbSqlSession.flushDeletes(DbSqlSession.java:598) ~[flowable-engine-common-6.6.0.jar:6.6.0]
at org.flowable.common.engine.impl.db.DbSqlSession.flush(DbSqlSession.java:365) ~[flowable-engine-common-6.6.0.jar:6.6.0]
at org.flowable.common.engine.impl.interceptor.CommandContext.flushSessions(CommandContext.java:211) ~[flowable-engine-common-6.6.0.jar:6.6.0]
at org.flowable.common.engine.impl.interceptor.CommandContext.close(CommandContext.java:69) ~[flowable-engine-common-6.6.0.jar:6.6.0]
at org.flowable.common.engine.impl.interceptor.CommandContextInterceptor.execute(CommandContextInterceptor.java:92) [flowable-engine-common-6.6.0.jar:6.6.0]
at org.flowable.common.engine.impl.interceptor.LogInterceptor.execute(LogInterceptor.java:30) [flowable-engine-common-6.6.0.jar:6.6.0]
at org.flowable.common.engine.impl.cfg.CommandExecutorImpl.execute(CommandExecutorImpl.java:56) [flowable-engine-common-6.6.0.jar:6.6.0]
at org.flowable.common.engine.impl.cfg.CommandExecutorImpl.execute(CommandExecutorImpl.java:51) [flowable-engine-common-6.6.0.jar:6.6.0]
at org.flowable.engine.impl.TaskServiceImpl.complete(TaskServiceImpl.java:212) [flowable-engine-6.6.0.jar:6.6.0]
at testcases.TaskIdentityLinkCountNegative.testUserIdentityLinkCountIsIncorrect(TaskIdentityLinkCountNegative.java:63) [test-classes/:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_272]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_272]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_272]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_272]
at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:513) [junit-platform-commons-1.2.0.jar:1.2.0]
at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:115) [junit-jupiter-engine-5.2.0.jar:5.2.0]
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$6(TestMethodTestDescriptor.java:170) [junit-jupiter-engine-5.2.0.jar:5.2.0]
at org.junit.jupiter.engine.execution.ThrowableCollector.execute(ThrowableCollector.java:40) ~[junit-jupiter-engine-5.2.0.jar:5.2.0]
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:166) [junit-jupiter-engine-5.2.0.jar:5.2.0]
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:113) [junit-jupiter-engine-5.2.0.jar:5.2.0]
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:58) [junit-jupiter-engine-5.2.0.jar:5.2.0]
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.lambda$executeRecursively$3(HierarchicalTestExecutor.java:113) ~[junit-platform-engine-1.2.0.jar:1.2.0]
at org.junit.platform.engine.support.hierarchical.SingleTestExecutor.executeSafely(SingleTestExecutor.java:66) ~[junit-platform-engine-1.2.0.jar:1.2.0]
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.executeRecursively(HierarchicalTestExecutor.java:108) ~[junit-platform-engine-1.2.0.jar:1.2.0]
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.execute(HierarchicalTestExecutor.java:79) ~[junit-platform-engine-1.2.0.jar:1.2.0]
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.lambda$executeRecursively$2(HierarchicalTestExecutor.java:121) ~[junit-platform-engine-1.2.0.jar:1.2.0]
at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:183) ~[na:1.8.0_272]
at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175) ~[na:1.8.0_272]
at java.util.Iterator.forEachRemaining(Iterator.java:116) ~[na:1.8.0_272]
at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801) ~[na:1.8.0_272]
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482) ~[na:1.8.0_272]
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472) ~[na:1.8.0_272]
at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:150) ~[na:1.8.0_272]
at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:173) ~[na:1.8.0_272]
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[na:1.8.0_272]
at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:485) ~[na:1.8.0_272]
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.lambda$executeRecursively$3(HierarchicalTestExecutor.java:121) ~[junit-platform-engine-1.2.0.jar:1.2.0]
at org.junit.platform.engine.support.hierarchical.SingleTestExecutor.executeSafely(SingleTestExecutor.java:66) ~[junit-platform-engine-1.2.0.jar:1.2.0]
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.executeRecursively(HierarchicalTestExecutor.java:108) ~[junit-platform-engine-1.2.0.jar:1.2.0]
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.execute(HierarchicalTestExecutor.java:79) ~[junit-platform-engine-1.2.0.jar:1.2.0]
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.lambda$executeRecursively$2(HierarchicalTestExecutor.java:121) ~[junit-platform-engine-1.2.0.jar:1.2.0]
at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:183) ~[na:1.8.0_272]
at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175) ~[na:1.8.0_272]
at java.util.Iterator.forEachRemaining(Iterator.java:116) ~[na:1.8.0_272]
at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801) ~[na:1.8.0_272]
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482) ~[na:1.8.0_272]
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472) ~[na:1.8.0_272]
at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:150) ~[na:1.8.0_272]
at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:173) ~[na:1.8.0_272]
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[na:1.8.0_272]
at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:485) ~[na:1.8.0_272]
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.lambda$executeRecursively$3(HierarchicalTestExecutor.java:121) ~[junit-platform-engine-1.2.0.jar:1.2.0]
at org.junit.platform.engine.support.hierarchical.SingleTestExecutor.executeSafely(SingleTestExecutor.java:66) ~[junit-platform-engine-1.2.0.jar:1.2.0]
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.executeRecursively(HierarchicalTestExecutor.java:108) ~[junit-platform-engine-1.2.0.jar:1.2.0]
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.execute(HierarchicalTestExecutor.java:79) ~[junit-platform-engine-1.2.0.jar:1.2.0]
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:55) ~[junit-platform-engine-1.2.0.jar:1.2.0]
at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:43) ~[junit-platform-engine-1.2.0.jar:1.2.0]
at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:170) ~[junit-platform-launcher-1.2.0.jar:1.2.0]
at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:154) ~[junit-platform-launcher-1.2.0.jar:1.2.0]
at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:90) ~[junit-platform-launcher-1.2.0.jar:1.2.0]
at com.intellij.junit5.JUnit5IdeaTestRunner.startRunnerWithArgs(JUnit5IdeaTestRunner.java:74) ~[junit5-rt.jar:na]
at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47) ~[junit-rt.jar:na]
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242) ~[junit-rt.jar:na]
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70) ~[junit-rt.jar:na]
Caused by: org.h2.jdbc.JdbcSQLException: Referential integrity constraint violation: "ACT_FK_TSKASS_TASK: PUBLIC.ACT_RU_IDENTITYLINK FOREIGN KEY(TASK_ID_) REFERENCES PUBLIC.ACT_RU_TASK(ID_) ('10')"; SQL statement:
delete from ACT_RU_TASK where ID_ = ? and REV_ = ? [23503-176]
at org.h2.message.DbException.getJdbcSQLException(DbException.java:344) ~[h2-1.3.176.jar:1.3.176]
at org.h2.message.DbException.get(DbException.java:178) ~[h2-1.3.176.jar:1.3.176]
at org.h2.message.DbException.get(DbException.java:154) ~[h2-1.3.176.jar:1.3.176]
at org.h2.constraint.ConstraintReferential.checkRow(ConstraintReferential.java:427) ~[h2-1.3.176.jar:1.3.176]
at org.h2.constraint.ConstraintReferential.checkRowRefTable(ConstraintReferential.java:444) ~[h2-1.3.176.jar:1.3.176]
at org.h2.constraint.ConstraintReferential.checkRow(ConstraintReferential.java:319) ~[h2-1.3.176.jar:1.3.176]
at org.h2.table.Table.fireConstraints(Table.java:909) ~[h2-1.3.176.jar:1.3.176]
at org.h2.table.Table.fireAfterRow(Table.java:927) ~[h2-1.3.176.jar:1.3.176]
at org.h2.command.dml.Delete.update(Delete.java:101) ~[h2-1.3.176.jar:1.3.176]
at org.h2.command.CommandContainer.update(CommandContainer.java:79) ~[h2-1.3.176.jar:1.3.176]
at org.h2.command.Command.executeUpdate(Command.java:254) ~[h2-1.3.176.jar:1.3.176]
at org.h2.jdbc.JdbcPreparedStatement.execute(JdbcPreparedStatement.java:199) ~[h2-1.3.176.jar:1.3.176]
at org.apache.ibatis.executor.statement.PreparedStatementHandler.update(PreparedStatementHandler.java:47) ~[mybatis-3.5.5.jar:3.5.5]
at org.apache.ibatis.executor.statement.RoutingStatementHandler.update(RoutingStatementHandler.java:74) ~[mybatis-3.5.5.jar:3.5.5]
at org.apache.ibatis.executor.SimpleExecutor.doUpdate(SimpleExecutor.java:50) ~[mybatis-3.5.5.jar:3.5.5]
at org.apache.ibatis.executor.BaseExecutor.update(BaseExecutor.java:117) ~[mybatis-3.5.5.jar:3.5.5]
at org.apache.ibatis.executor.CachingExecutor.update(CachingExecutor.java:76) ~[mybatis-3.5.5.jar:3.5.5]
at org.apache.ibatis.session.defaults.DefaultSqlSession.update(DefaultSqlSession.java:197) ~[mybatis-3.5.5.jar:3.5.5]
... 66 common frames omitted
```

**Additional context**
Seen in Flowable 6.6.0 using H2 and Postgres database.