[FLINK-18429][DataStream API] Make CheckpointListener.notifyCheckpointAborted(checkpointId) a default method.

[StephanEwen]

Purpose / Brief change log

This PR prevents breaking existing user programs by making the newly added method CheckpointListener.notifyCheckpointAborted(checkpointId) a default method.

In addition, this PR upgrades the stability of CheckpointListener and CheckpointedFunction to @Public to reflect that we should not break them and make sure this is caught by our tooling.

Original Confusion about (Not) Having a Default Method

(Copying the description from the JIRA issue here)

There was confusion about this originally, going back to a comment by myself suggesting this should not be a default method, incorrectly thinking of it as an internal interface: #8693 (comment)

See clarification email on the mailing list:

About the "notifyCheckpointAborted()":

When I wrote that comment, I was (apparently wrongly) assuming we were talking about
an internal interface here, because the "abort" signal was originally only intended to cancel
the async part of state backend checkpoints.

I just realized that this is exposed to users - and I am actually with Thomas on this one. 
he "CheckpointListener" is a very public interface that many users implement. The fact that
it is tagged "@PublicEvolving" is somehow not aligned with reality. So adding the method
here will in reality break lots and lots of user programs.

I think also in practice it is much less relevant for user applications to react to aborted checkpoints.
Since the notifications there can not be relied upon (if there is a task failure concurrently) users
 always have to follow the "newer checkpoint subsumes older checkpoint" contract, so the abort
method is probably rarely relevant.

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): yes
• 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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Automated Checks

Last check on commit cf0c7ec (Wed Jun 24 16:20:19 UTC 2020)

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[pnowojski]

Thanks for providing a fix and resolving the confusion :)

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CI report:

• cf0c7ec Azure: PENDING

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