[FLINK-19109][task] Ignore isLoopRunning in MailboxExecutor.isIdle

[rkhachatryan]

What is the purpose of the change

Currently, MailboxExecutor.isIdle checks whether the mailbox loop is running or not.
If not, ContinuousFileReaderOperator continues file processing loop without going through the mailbox.

However, the mailbox can still contain (and receive new) mails (e.g. from timers) that should be processed.
In particular, this check currently prevents periodic watermarks from being emitted.

This PR simply removes this check.

Verifying this change

• Added FileReadingWatermarkITCase
• Existing tests cover the rest of ContinuousFileReaderOperator (e.g. ContinuousFileProcessingTest)

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
• 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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Last check on commit 6da9dbc (Wed Sep 02 11:12:30 UTC 2020)

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[rkhachatryan]

I've added 2 fixup commits, can you take a look @pnowojski, @alpinegizmo?

[alpinegizmo]

LGTM.

[pnowojski]

Could you squash the fixup commits?

[rkhachatryan]

Could you squash the fixup commits?

Done.