[FLINK-21028][task] Do not interrupt the source thread on stop with savepoint

[pnowojski]

Currently stop with savepoint relies on the EndOfPartitionEvents propagation and performs
clean shutdown after the stop with savepoint (which can produce some records to process after
the savepoint while stopping). If we interrupt source thread, we might leave the newtork stack
in an inconsitent state. So, if we want to relay on the clean shutdown, we can not interrupt
the source thread.

Verifying this change

This change add new test to add coverage for this bug fix.This change is a trivial rework / code cleanup without any test coverage.

I have also manually tested, that if stop with savepoint is stuck waiting on a source that ignores clean #cancel() call, the job can be still manually cancelled.

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• 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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[rkhachatryan]

Thanks for the fix and the investigation @pnowojski .
I have some questions regarding the test.
Production code changes LGTM.

[rkhachatryan]

LGTM

[pnowojski]

Thanks for the review @rkhachatryan . Azure is green. Merging the fix I will also prepare backport PRs