[FLINK-28820][Connector/Pulsar] Improve the writing performance for PulsarSink

[syhily]

What is the purpose of the change

PulsarSink is slow when we use the At-Least-Once or Exactly-Once. This is because PulsarSink uses MailboxExecutor to write messages. A lot of useless context switches and locks acquire happened when using this writing policy. We should drop MailboxExecutor and send messages directly for better performance.

Brief change log

• Deprecated useless PulsarSinkOptions.PULSAR_MAX_PENDING_MESSAGES_ON_PARALLELISM option since Pulsar client has its own send rate limit by memory.
• Remove the use of MailboxExecutor in PulsarWriter.

Verifying this change

This change is already covered by existing tests, such as PulsarWriterTest.

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, 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? (not applicable)

[flinkbot]

CI report:

• 6004175 Azure: SUCCESS
• 476088c Azure: PENDING

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• @flinkbot run azure re-run the last Azure build

[tisonkun]

LGTM. Merging...