Update SplittableDoFn implementation within Python to use ProcessContinuation instead of defer_remainder/defer_status on the RestrictionTracker

The current API for a SplittableDoFn doesn't follow the API in https://docs.google.com/document/d/1h_zprJrOilivK2xfvl4L42vaX4DMYGfH1YDmi-s_ozM/edit?usp=sharing

Specifically the intent was that a ProcessContinuation was to be returned as the last result for a a SplittableDoFn signalling the users intent as to whether the restriction is done or whether it should be resumed after a certain amount of time.

This requires updating the Python SDK implementation to not rely on RestrictionTracker.defer_remainder and RestrictionTracker.defer_status