[FLINK-9904]Allow users to control MaxDirectMemorySize

[tianchen92]

What is the purpose of the change

This PR is related to FLINK-9904. Make MaxDirectMemorySize configurable via flink-conf.yaml.

Brief change log

• Make MaxDirectMemorySize configurable.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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, Yarn/Mesos, ZooKeeper: (don't know)
• 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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[tianchen92]

@tillrohrmann hi, could you please help review this PR? thanks!

[klion26]

@tianchen92 thanks for your contribution, Could you please clear the last merge commit? you can use git rebase.