Periodically commit contents of ref in processingAndCommitting

[amohrland]

KAYAK-3790 Periodically commit contents of ref in processingAndCommitting. Draft for now, since even if the logic updates are sound, it doesn't yet do what we ultimately want, because the ref's offsets are still populated solely by encountering events. Still need to find a way to plumb looked-up offsets into the ref before processing starts.