Create a callback triggered at the end of a batch in spark 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-3471](https://issues.apache.org/jira/browse/BEAM-3471). Original Jira may contain additional context.
Reported by: echauchot.

Start by locating the Spark runner and its batch lifecycle entry point; the issue does not name a file, test, or existing callback. Review the imported BEAM-3471 Jira issue for additional context, then define the callback's lifecycle and verify that end-of-batch processing is covered by runner tests.

Written by the indexing model from the issue text.