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

Merge EventIterable and JcrListener to EventGenerator to simplify the code
and to use just a single list of events as the event queue.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1561710 13f79535-47bb-0310-9956-ffa450edef68