Space Persisters need their own thread pool

Summary: This hasn't proven to be a problem yet, but probably will be. At the moment, we allow an arbitrary number of Space Persisters to try to write simultaneously. That's fine unless the DB communications load completely; if they do, the Space Persisters will eventually starve the global threadpool.

Not entirely obvious what the correct response it, but we probably need to detect this situation, and apply back-pressure against changes coming from the clients until it drains.