[FLINK-18728][network] Make initialCredit of RemoteInputChannel final

[wsry]

What is the purpose of the change

The filed initialCredit of RemoteInputChannel is set only once and can be accessed by multi threads. This patch makes the filed final and moves the initialization to the constructor of RemoteInputChannel to avoid potential thread safety issues.

Brief change log

• Make initialCredit of RemoteInputChannel final

Verifying this change

This change is already covered by existing tests.

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 a031dfe (Mon Jul 27 08:17:25 UTC 2020)

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• ❓ 1. The [description] looks good.
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CI report:

• 370e0fc Azure: SUCCESS
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[zhijiangW]

Thanks for the improvement @wsry , the codes look good to me. Only left one comment for further improvement.

[zhijiangW]

Thanks for the updates and I left some other tiny comments.

[zhijiangW]

Thanks for addressing my comments, LGTM!

[wsry]

The failed e2e test should be irrelevant, all tests passed on my private pipeline: https://dev.azure.com/kevin-flink/flink/_build/results?buildId=56&view=results