Java SDK Harness throws IllegalStateException with user states in finishBundle

```

Error message from worker: java.lang.IllegalStateException: java.lang.IllegalStateException: Attempt
to register more than 65535 parties for java.util.concurrent.Phaser@16e4ff69[phase = 0 parties = 65535
arrived = 0]
org.apache.beam.fn.harness.state.FnApiStateAccessor.finalizeState(FnApiStateAccessor.java:498)
org.apache.beam.fn.harness.FnApiDoFnRunner.finishBundle(FnApiDoFnRunner.java:1204)
org.apache.beam.fn.harness.data.PTransformFunctionRegistry.lambda$register$0(PTransformFunctionRegistry.java:108)
org.apache.beam.fn.harness.control.ProcessBundleHandler.processBundle(ProcessBundleHandler.java:302)
org.apache.beam.fn.harness.control.BeamFnControlClient.delegateOnInstructionRequestType(BeamFnControlClient.java:173)
org.apache.beam.fn.harness.control.BeamFnControlClient.lambda$processInstructionRequests$0(BeamFnControlClient.java:157)
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
java.lang.Thread.run(Thread.java:748)
Caused
by: java.lang.IllegalStateException: Attempt to register more than 65535 parties for java.util.concurrent.Phaser@16e4ff69[phase
= 0 parties = 65535 arrived = 0]
java.util.concurrent.Phaser.doRegister(Phaser.java:438)
java.util.concurrent.Phaser.register(Phaser.java:591)
org.apache.beam.fn.harness.control.ProcessBundleHandler$BlockTillStateCallsFinish.handle(ProcessBundleHandler.java:712)
org.apache.beam.fn.harness.state.BagUserState.asyncClose(BagUserState.java:123)
org.apache.beam.fn.harness.state.FnApiStateAccessor.finalizeState(FnApiStateAccessor.java:492)
...
8 more

```

This is probably caused by concurrently accessing too many states (too many state keys).

Imported from Jira [BEAM-10539](https://issues.apache.org/jira/browse/BEAM-10539). Original Jira may contain additional context.
Reported by: yichi.