Thread pool for persisting to Object Store

I plan to look at removing duplication from `BrooklynMementoPersisterToObjectStore`, but not in this pull request.
Agree worth a comment.
Correct that currently just called by a single-threaded scheduled executor.

For the unstarted persistence task, that's a bit trickier. We'd need to merge the deltas together so that we do persist all of the entities needing persisted. That behaviour of only persisting most recent is what used to happen in the overly complicated async writer class.