[FLINK-18935] Reject CompletedOperationCache.registerOngoingOperation if cache is shutting down

[tillrohrmann]

What is the purpose of the change

This commit changes the CompletedOperationCache so that it rejects registerOngoingOperation calls
with an IllegalStateException if the cache is already shutting down. This ensures that now new
operation will be enqueued while waiting for the previous operations to complete.

Verifying this change

• Added CompletedOperationCacheTest.testCannotAddOperationAfterClosing and CompletedOperationCacheTest.testCanGetOperationResultAfterClosing

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, Kubernetes/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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Last check on commit 8e3849d (Thu Aug 13 08:56:12 UTC 2020)

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[rmetzger]

The change looks good to me. +1 to merge.

[tillrohrmann]

Thanks for the review @rmetzger. Merging this PR now.