[FLINK-20431][connector/kafka] Fix the offsets commit in KafkaSourceReader when the SplitFetcher shuts down.

[becketqin]

What is the purpose of the change

The patch fixes a bug that the KafkaPartitionSplitFetcher does not finish the pending offsets commit when the contained KafkaConsumer exits. This is actually a bug in KafkaConsumer but we need to handle it for now.

The previous test tries to play a trick so that the unit test passes. We should have handled this correctly in the KafkaPartitionSplitFetcher instead.

The patch also fixes the reported instability of the KafkaSourceReaderTest.testCommitOffsetsWithoutAliveFetchers().

Brief change log

The patch tracks all the pending async commit and commit them when the KafkaPartitionSplitFetcher exits.

Verifying this change

The following unit tests have been added to verify the change.
KafkaPartitionSplitReaderTest.testFinishPendingOffsetsCommitOnClose()

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? (not applicable)

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Automated Checks

Last check on commit 5893b7c (Wed Dec 02 23:50:45 UTC 2020)

Warnings:

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CI report:

• 5893b7c Azure: FAILURE

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[MartijnVisser]

@becketqin If this code change is still relevant, please open the PR in https://github.com/apache/flink-connector-kafka