[FLINK-20194][connector/kafka] Fix the KafkaSourceReader offset commit.

[becketqin]

What is the purpose of the change

The patch fixes the offsets commit in the following cases:
1. The SplitFetcher has exited.
2. The offsets to be committed is empty.
3. The offsets commit for finished splits.

Brief change log

• Fix the SingleThreadFetcherManager to get the running fetchers correctly.
• Change SourceReaderBase.onSplitFinished() to take a map of SplitId -> SplitState.
• Fix Kafka offset commit to coorectly handle the following cases:
  1. The SplitFetcher has exited.
  2. The offsets to be committed is empty.
  3. The offsets commit for finished splits.

Verifying this change

The unit tests has been added to KafkaSourceReaderTest.

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): (yes, the argument passed to SourceReaderBase.onSplitFinish() has changed)
• 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 92b26cf (Thu Nov 19 17:56:17 UTC 2020)

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CI report:

• 78807d6 Azure: SUCCESS

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[becketqin]

@flinkbot run azure

[StephanEwen]

I looked at the API change of the base connector, and it seems reasonable.
We need to make sure this is picked back to the release-1.11 branch together with the other source api changes, though.

[becketqin]

@flinkbot run azure

[becketqin]

merged to master.
4bde1c9