#2 Clients should be notified of config changes

[Joseph Hopfield]

Clients need to be told when engines or replicas are added or deleted at the server. Currently only polls.

Main issue is that the notification mustn't block - one server must handle thousands of replicas, so it could easily get out of hand.

At the same time, its a must: I did a detach of a replica, then deleted the whole tree at the detached site. Since it wasn't really detached yet (a minute or two until next poll), the tree removal PROPAGATED.

Joe