[FLINK-21935] Remove 'state.backend.async' option

[StephanEwen]

What is the purpose of the change

Removes the state.backend.async option and makes all state snapshots asnchronous by default.

Brief change log

• Removes the flag for async snapshots from the HashMapStateBackend and related examples and tests. This code was not released before, so this change is not breaking any backwards compatibility.
• Ignores the flag for async snapshots in the MemoryStateBackend and FsStateBackend and always uses async snapshots. This change does not break any API. Methods and constructors are changed to ignore the flag, JavaDocs updated.
• Deprecates the ConfigOption for state.backend.async.

Verifying this change

• Existing tests are adjusted.

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: (no)
• 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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Last check on commit 0c7ab0b (Sat Aug 28 11:12:07 UTC 2021)

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CI report:

• de1587a UNKNOWN
• 0c7ab0b Azure: SUCCESS

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[Myasuka]

I think descriptions of MemoryStateBackend and FsStateBackend from state_backend.py should also be updated

[sjwiesman]

@Myasuka I've updated the pydoc in state_backend.py in #15441

[StephanEwen]

@sjwiesman From what I see, you updated the pydoc to mention that the old state backends are deprecated. I think @Myasuka was referring to update the docs that the async flag is no longer taking any effect and that snapshots are always asynchronous.

[Myasuka]

@sjwiesman Yes, what actually I meant is those pydoc about is_using_asynchronous_snapshots is actually out of date after this PR.

[carp84]

I suggest we also deprecate (or directly remove) NestedMapsStateTable and NestedStateMap classes since now there are actually no way to use them through any configuration setting, maybe deprecate first to reduce the changes this time.

[StephanEwen]

@carp84 Yes, I would do this when removing the actual internal synchronous checkpointing code. There is even a bit more than those two classes. This first change is mainly to reduce API and config surface.

[StephanEwen]

Merging this with an adjustment of the pydocs...