withNewSession can cause 'Active connection is required' exception under particular scenarios

### Task List

- [x] Steps to reproduce provided
- [x] Stacktrace (if present) provided
- [x] Example that reproduces the problem uploaded to Github
- [x] Full description of the issue provided (see below)

### Steps to Reproduce

1. git clone clone https://github.com/davidbairdala/active-connection-required-grails-bug.git
2. cd to 'active-connection-required-grails-bug'
3. grails run-app
4. Click on the links to demonstrate the failure and success scenarios

### Expected Behaviour

The links marked as 'Will fail' should succeed without an exception

### Actual Behaviour

Links marked as 'Will fail' will throw an exception with the message 'An active connection is required'

It seems that if code inside a withNewSession block calls a method on another service that is marked as Transactional then the exception will occur. It doesn't make any difference if the second service method actually makes use of the database or not.

### Stack trace
```
java.lang.reflect.InvocationTargetException: null
at org.grails.core.DefaultGrailsControllerClass$ReflectionInvoker.invoke(DefaultGrailsControllerClass.java:211)
at org.grails.core.DefaultGrailsControllerClass.invoke(DefaultGrailsControllerClass.java:188)
at org.grails.web.mapping.mvc.UrlMappingsInfoHandlerAdapter.handle(UrlMappingsInfoHandlerAdapter.groovy:90)
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:967)
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:901)
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861)
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
at org.springframework.boot.web.filter.ApplicationContextHeaderFilter.doFilterInternal(ApplicationContextHeaderFilter.java:55)
at org.grails.web.servlet.mvc.GrailsWebRequestFilter.doFilterInternal(GrailsWebRequestFilter.java:77)
at org.grails.web.filters.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:67)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.IllegalArgumentException: Active Connection is required
at org.springframework.util.Assert.notNull(Assert.java:134)
at org.springframework.jdbc.datasource.ConnectionHolder.getConnection(ConnectionHolder.java:150)
at org.grails.orm.hibernate.GrailsHibernateTemplate.executeWithNewSession(GrailsHibernateTemplate.java:173)
at org.grails.orm.hibernate.AbstractHibernateDatastore.withNewSession(AbstractHibernateDatastore.java:360)
at org.grails.orm.hibernate.AbstractHibernateGormStaticApi.withNewSession(AbstractHibernateGormStaticApi.groovy:60)
at org.grails.datastore.gorm.GormEntity$Trait$Helper.withNewSession(GormEntity.groovy:1025)
at no.active.connection.TestService.$tt__createFail1(TestService.groovy:14)
at grails.gorm.transactions.GrailsTransactionTemplate$2.doInTransaction(GrailsTransactionTemplate.groovy:94)
at org.springframework.transaction.support.TransactionTemplate.execute(TransactionTemplate.java:133)
at grails.gorm.transactions.GrailsTransactionTemplate.execute(GrailsTransactionTemplate.groovy:91)
at no.active.connection.TestController.$tt__willFail1(TestController.groovy:15)
at grails.gorm.transactions.GrailsTransactionTemplate$2.doInTransaction(GrailsTransactionTemplate.groovy:94)
at org.springframework.transaction.support.TransactionTemplate.execute(TransactionTemplate.java:133)
at grails.gorm.transactions.GrailsTransactionTemplate.execute(GrailsTransactionTemplate.groovy:91)
... 14 common frames omitted

```
### Environment Information

- **Operating System**: Linux (Ubuntu 18.04), Mac OS X
- **GORM Version:** 6.1.11-RELEASE
- **Grails Version (if using Grails):** 3.3.9
- **JDK Version:** OpenJDK 1.8.0_191

### Example Application

https://github.com/davidbairdala/active-connection-required-grails-bug.git