[FLINK-12967][runtime] Change the processing that the input reaches the end to comply with the InputSelectable contract

[sunhaibotb]

What is the purpose of the change

This pull request changes the processing that the input reaches the end to comply with the following InputSelectable contract.

• When the input being read reaches the end, the runtime must call nextSelection() to determine the next input to be read.

Brief change log

• Uses invoking InputSelectable#nextSelection() to replace the assignment on the input selection in StreamTwoInputSelectableProcessor#checkFinished() . The input selection should been switched in endInput() by the operator.

Verifying this change

This change is already covered by existing tests, such as StreamTaskSelectiveReadingTest, StreamTaskSelectiveReadingITCase.

Does this pull request potentially affect one of the following parts:

• 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)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, 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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[pnowojski]

LGTM. Thanks for the change. I will merge it with green travis :)

[sunhaibotb]

Thanks for the review. The travis is already green. @pnowojski