[FLINK-20654][checkpointing] Decline checkpoints until restored channel state is consumed

[rkhachatryan]

What is the purpose of the change

In scenarios with multiple inputs (e.g. co-group; not union) one input may receive a
checkpoint barrier while the second input is still restoring state. This (previous)
state is currently not included into the snapshot, which therefore will be incomplete.

Brief change log

• Disable alignment timeout by default in UnalignedCheckpointITCase
• Decline checkpoint if state of the input gate was not yet consumed

Verifying this change

• Added unit test SingleInputGateTest#testCheckpointsDeclinedUnlessStateConsumed.
• Existing UnalignedCheckpointITCase un-ignored - without the change it fails once per ~10 runs

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? no

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Automated Checks

Last check on commit c020556 (Fri May 28 06:58:28 UTC 2021)

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• ❓ 1. The [description] looks good.
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• ❓ 3. Needs [attention] from.
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• ❓ 5. Overall code [quality] is good.

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CI report:

• c020556 Azure: SUCCESS

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[pnowojski]

LGTM % one comment and compilation failure