[FLINK-20232][python] Handle object reuse properly in the operators of Python DataStream API.

[shuiqiangchen]

What is the purpose of the change

Currently, it doesn't consider object reuse in the operators of Python DataStream API.

Brief change log

• Added object reuse check in PythonTimestampAndWatermarkOperator before buffering input values.

Verifying this change

This change has test case covered by e2e test for Python DataStream API.

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/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? (not documented)

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Automated Checks

Last check on commit 48dd674 (Thu Nov 19 07:11:31 UTC 2020)

Warnings:

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• ❓ 1. The [description] looks good.
• ❓ 2. There is [consensus] that the contribution should go into to Flink.
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• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

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CI report:

• 48dd674 Azure: SUCCESS

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[dianfu]

LGTM.