[FLINK-8517] fix missing synchronization in TaskEventDispatcher

[NicoK]

What is the purpose of the change

The TaskEventDispatcher was missing synchronization accessing the registeredHandlers field for the new subscribeToEvent() and publish() methods. This was causing the StaticlyNestedIterationsITCase.testJobWithoutObjectReuse() test to sporadically fail (reproducible after running a couple of times).

Please merge into master and release-1.5 after accepting.

Brief change log

• add synchronization around TaskEventDispatcher#subscribeToEvent()'s access to registeredHandlers
• add synchronization around TaskEventDispatcher#publish()'s access to registeredHandlers

Verifying this change

This change is already covered by existing tests (indirectly), such as StaticlyNestedIterationsITCase.testJobWithoutObjectReuse(). I ran it almost 24000 times and could not reproduce it anymore with the change

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[NicoK]

FYI: test failure is unrelated and relates to #5606 which apparently does not (completely) fix the issue