[FLINK-19833] Rename Sink API Writer interface to SinkWriter

[guoweiM]

What is the purpose of the change

This makes it more consistent with SourceReader.

Brief change log

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

.

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 applicable)

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Automated Checks

Last check on commit 9d5d7fb (Mon Nov 02 13:55:57 UTC 2020)

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• No documentation files were touched! Remember to keep the Flink docs up to date!

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CI report:

• 9d5d7fb Azure: FAILURE

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[kl0u]

LGTM! I will also let it run on AZP to be sure and then merge.