[FLINK-22053][core] Allow NumberSequenceSource to have less splits than parallelism.

[AHeise]

What is the purpose of the change

NumberSequenceSource causes fatal exception when less splits than parallelism.

Brief change log

• Allow NumberSequenceSource to have less splits than parallelism.
• Merged both ITCases.

Verifying this change

Added IteratorSourcesITCase#testLessSplitsThanParallelism.

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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Automated Checks

Last check on commit 90e7028 (Wed Mar 31 08:52:28 UTC 2021)

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• ❓ 1. The [description] looks good.
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• ❓ 3. Needs [attention] from.
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[StephanEwen]

Looks good to me with few minor comments.

[StephanEwen]

Can we use DataStream.executeAndCollect() instead?
That is the main code users also use, let's give it as much exposure in tests as possible.

[AHeise]

I have also changed it in the original test.

[StephanEwen]

10ms is pretty long here, would 2ms also do it?

[AHeise]

I have slighty increased it instead, but only apply it to the first element now.

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CI report:

• 910a8d4 Azure: SUCCESS

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[StephanEwen]

LGTM. +1 to merge this.

[StephanEwen]

Merging this...

(I need to build on top of this).