Rework Space creation to be more atomic

Summary: Restructure Space creation to be more atomic and FP-friendly.

We're currently assigning the SpaceID way up in SpaceManager, then going down the stack through SpaceCreator to SpaceManagerPersister (across Actor boundaries) to actually try writing the RDB row, so if something goes wrong at the MySQL level (eg, the OID is already allocated because we reset Cassandra but not MySQL) we have no good way to recover save to try repeatedly allocating.

Really, this is a poster child to the general problem that the whole structure needs a rethink, to figure out how to ingregate CE3 semantics over the Akka/Pekko architecture.