Thread pool for persisting to Object Store

I plan to look at removing duplication from `BrooklynMementoPersisterToObjectStore`, but not in this pull request.
is there a danger that we might have many threads backing up here?  i think not, that persistence is invoked by a single thread, but if we start doing a lot of out-of-band `persistNow` we might.

worth a comment that an "if-unstarted-persistence-task-is-pending-then-just-wait-on-him-rather-than-submit-another" might be wanted in future?