OAK-1332: Large number of changes to the same node can fill observation queue

Postpone generation of NODE_ADDED and NODE_REMOVED events to when the
added or removed node gets diffed. This way only a single NODE_ADDED or
NODE_REMOVED event instance will be in the in the event queue at any
given time.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1561798 13f79535-47bb-0310-9956-ffa450edef68