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

Merge EventContext and EventImpl into QueueingHandler to reduce the number
of and size of objects kept around for event generation.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1562321 13f79535-47bb-0310-9956-ffa450edef68