[FLINK-13275] Fix race condition in finishTask().

[kl0u]

What is the purpose of the change

This PR fixes the race condition that may manifest itself during stop-with-savepoint, as described in the related JIRA.

Verifying this change

This change is already covered by existing tests, such as SourceStreamTaskTest.finishingIgnoresExceptions().

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

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

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Automated Checks

Last check on commit 0e37a66 (Tue Aug 06 15:43:54 UTC 2019)

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[kl0u]

R @aljoscha

[flinkbot]

CI report:

• 6b5e837 : SUCCESS Build

[aljoscha]

Maybe @1u0 also want's to take a look

[1u0]

I've left a minor notice.
Otherwise LGTM.

[1u0]

Personally, I'd remove this comment block, as it may be irrelevant or become outdated.
The comment near the isFinished field already has some explanation wrt exceptions.

[kl0u]

Thanks for the reviews. I will merge.