[FLINK-35051][checkpoint] Treat handling of unaligned checkpoint barrier as the high priority mail

[1996fanrui]

What is the purpose of the change

[FLINK-35051][checkpoint] Treat handling of unaligned checkpoint barrier as the high priority mail

Brief change log

• [FLINK-35051][checkpoint] Treat handling of unaligned checkpoint barrier as the high priority mail
• Taking high priority mail first when taking mail from TaskMailboxImpl even if one batch is created.

Verifying this change

This change added tests and can be verified as follows:

(example:)

• Added testPutHighPriorityMailDuringTakingFromBatch

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, introducing a check for hasNewHighPriorityMail, it's a volatile field, and it's false in most of the time.
• 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

[flinkbot]

CI report:

• 4494be34eb61a60814637e15a03341698d60c808 Azure: SUCCESS

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