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

Rename ChangeHandler to EventHandler for better consistency with the
related interfaces, and merge .handler to the main .observation package
now that the oak-jcr -specific code is no longer there.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1562531 13f79535-47bb-0310-9956-ffa450edef68