[FLINK-23025][upsert-kafka] Fix upsert-kafka produce duplicates when …

[fsk119]

…enable object reuse

What is the purpose of the change

Fix upsert-kafka produce duplicates when enable object reuse.

Verifying this change

• Add tests in BufferedUpsertSinkFunctionTest to verify the results if enable object reuse.

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• 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)
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• 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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Last check on commit fdfe95d (Tue Jun 22 12:09:46 UTC 2021)

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[wuchong]

Thanks for the fixing. I only left 2 minor comments. Others looks good to me.

[wuchong]

LGTM

[wuchong]

@fsk119 could you also open a check-pick PR for master branch ?

[fsk119]

cc #16257