Dismantle remainder of SpaceManager

(Incomplete Feature, To be Fixed , Priority: High, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Scalability
Summary: We've already moved the main Space message routing out into ClusterSharding, which takes most of the weight off of it. But the remainder should probably get moved into UserSession.
This is high-priority, but not actually critical-path for getting clustering working.
The one trick is going to be making sure that all of the UserSessions can share the common pool of SpaceManagerPersisters. I believe that's straightforward, but it needs some real rewriting.