[FLINK-18883][python] Support reduce() operation for Python KeyedStream.

[hequn8128]

What is the purpose of the change

This pull request adds reduce operation and reduce functions for Python DataStream API.

Brief change log

• Adds reduce operation on Python KeyedStream.
• Adds Python ReduceFunction.
• Add tests in test_data_stream.py

Verifying this change

This change added tests and can be verified as follows:

• Added integration tests in test_data_stream.py

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? (yes)
• If yes, how is the feature documented? (PythonDocs)

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Automated Checks

Last check on commit eae6803 (Tue Aug 11 05:33:22 UTC 2020)

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• ❓ 1. The [description] looks good.
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• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

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CI report:

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[shuiqiangchen]

Hi @hequn8128, thanks for your PR, the code looks pretty good to me. I think the flink-python/dev/glibc_version_fix.h should not be included in the PR since it has no change.

[hequn8128]

@shuiqiangchen I have removed the file and updated the PR. Thanks.