[FLINK-21505] Enforce common/unified savepoint format at operator level

[aljoscha]

Before, we were relying on the fact that all keyed backends would use the same strategy for savepoints.

Now, we're forcing them at the API level to provide a SavepointResources that we can then use to create a unified savepoint using SavepointSnapshotStrategy.

Verifying this change

This is a refactoring and is covered by existing tests.

Does this pull request potentially affect one of the following parts:

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• 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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CI report:

• 58fdc4f Azure: FAILURE Azure: FAILURE Azure: FAILURE

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[dawidwys]

Generally speaking I like the idea. I think it is a nice way to ensure all state backends produce a common unified format.

I will go over the PR and do a proper review.

[dawidwys]

Do we want to hardcode ASYNCHRONOUS here?

[dawidwys]

Good job! I really like the flow now!

[aljoscha]

@flinkbot run azure

[aljoscha]

@flinkbot run azure

[dawidwys]

I think the problem with azure is that it is not rebased on the fixed master. I have rebased the PR on current master and I am running it on my private azure: https://dev.azure.com/wysakowiczdawid/Flink/_build/results?buildId=707&view=results