Revision 389941

- Since we now have per destination usage managers, I updated the MessageStore API so that each destination's store can be maded aware of the manager for that store. 
  Some stores like the journal hold messages around and use the usage manager to know when it should flush to disk.
- Moved alot of the message reference counting logic from the PrefetchSubscription up to it's subclasses since they all seem to do it just a slightly different way.
  I think it makes easier to see now how the usage manager is affected by operations.
- I fixed and verifed that the keepDurableSubsActive=true option actually works.  I think we should make this the default setting since it
  make recovery safer.  Once we have a better spooling implementation we can turn if off again.