[FLINK-18421][checkpointing] Fix logging of RejectedExecutionException logging

[rkhachatryan]

What is the purpose of the change

If CheckpointCoordinator is shutting down then RejectedExecutionExceptions are possible. They are logged separately at DEBUG level.
However, they can be wrapped into CompletionException, in which case they are logged fully at WARN level, which fails e2e tests.

This PR fixes it by searching for RejectedExecutionException in cause chain of the exception.

Verifying this change

This change is a trivial rework; e2e tests already cover this issue.

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? no

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Automated Checks

Last check on commit 26cbc62 (Wed Jul 22 16:11:16 UTC 2020)

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[pnowojski]

Thanks for the fix, merging.