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

Break the references from Event instances to the originating event handler,
so that client code that separately queues events won't unnecessarily hold
on to too much memory.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1562562 13f79535-47bb-0310-9956-ffa450edef68