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

Extract the shared state of all observation events originating from the
same commit to a separate EventContext object. This helps reduce the
memory overhead of a large queue of events.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1561246 13f79535-47bb-0310-9956-ffa450edef68