[FLINK-15467][task] Wait for sourceTaskThread to finish before exiting from StreamTask.invoke

[rkhachatryan]

What is the purpose of the change

Wait for the LegacySourceFunctionThread to finish when cancelling the task.
This prevents premature freeing of resources, particularly LibraryCache (see FLINK-15467 discussion).

Implemented by waiting in SourceStreamTask.invoke() for the sourceThread to complete.

There were several existing tests failures (listed below) which were fixed by completing the future if thread isn't alive; or by returning a completed future if a task is failing.

Verifying this change

• Added unit test: SourceStreamTaskTest.testWaitsForSourceThreadOnCancel
• Tested manually on a local cluster
• Existing tests covering different scenarios of task exit and cancellation: InterruptSensitiveRestoreTest, StreamTaskTest.testEarlyCanceling, FastFailuresITCase

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, Kubernetes/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? no

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Automated Checks

Last check on commit 246e726 (Mon Jul 27 21:43:39 UTC 2020)

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[pnowojski]

I think it either LGTM or almost LGTM % two questions :)

[rkhachatryan]

Thanks for the great feedback on these PR series @pnowojski :)