[FLINK-20097][checkpointing] Race conditions in InputChannel.ChannelStatePersister

[rkhachatryan]

What is the purpose of the change

Fix race conditions in InputChannel.ChannelStatePersister
(please see FLINK-20097 for more details).

Verifying this change

• Added two unit tests (ChannelStatePersisterTest)
• UnalignedCheckpointITCase untouched for now (created a follow-up FLINK-20103)

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): yes (checkForBarrier)
• 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? no

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Automated Checks

Last check on commit d95bf0f (Wed Nov 11 18:09:19 UTC 2020)

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[AHeise]

LGTM % minus squashing some (all?) commits.

[rkhachatryan]

Thanks for reviewing.
I've added one more unit test and squashed the commits (except the refactoring).

[AHeise]

LGTM. I wait for AZP before merging.