[FLINK-24347][connectors/kafka] Keep idle source readers if paralleli…

[fapaul]

…sm is higher than partitions in KafkaSource

What is the purpose of the change

Before this commit the enumerator signaled the leftover source readers
without a partition to finish. This caused that checkpointing was not
possible anymore because it is only supported if all tasks are running
or FLIP-147 is enabled.

Brief change log

• Only send the signal to finish if the Source is executed in BOUNDED mode.

Verifying this change

• Changed the existing test case for parallelism > partitions to assert that a checkpoint is taken

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, 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 9408d97 (Tue Sep 21 12:46:13 UTC 2021)

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CI report:

• 73c07ed UNKNOWN
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[StephanEwen]

Looks good, +1 to merge this.

Minor comment inline.