[FLINK-17820][task][checkpointing] Don't flush channel state to disk explicitly

[rkhachatryan]

What is the purpose of the change

See FLINK-17820 for full problem description. In short, Flink ignores state.backend.fs.memory-threshold and always creates a file per subtask per checkpoint.

This PR wraps OutputStream in ChannelStateWriter into a non-flushing one; so that data can be flushed from DataOutputStream, but not to disk.
This solution was chosen:

• over changing the original stream because of lower risk
• over not flushing in ChannelStateWriter because that could lead to data loss in DataOutputStream

Verifying this change

Added ChannelStateCheckpointWriterTest.testSmallFilesNotWritten.

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, Kubernetes/Yarn/Mesos, ZooKeeper: yes
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicabl

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Automated Checks

Last check on commit 8d01ba8 (Fri Oct 16 10:55:21 UTC 2020)

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CI report:

• 8d01ba8 Azure: FAILURE

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[StephanEwen]

I left some comments on the jira issue.

[rkhachatryan]

build failure is unrelated: https://issues.apache.org/jira/browse/FLINK-17817

[rkhachatryan]

Obsoleted by #12332