Create a callback triggered at the end of a batch in flink runner

In the future we might add new features to the runners for which we might need to do some processing at the end of a batch. Currently there is not unique place (a callback) to add this processing.

Imported from Jira [BEAM-3472](https://issues.apache.org/jira/browse/BEAM-3472). Original Jira may contain additional context.
Reported by: echauchot.

Start by reviewing the Flink runner's batch lifecycle and the additional context in Jira BEAM-3472. Determine the appropriate end-of-batch integration point and define the callback's behavior; the work is done when future processing can be registered there.

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