OAK-960 -  Provide an interceptor for SessionOperations

Replacing the use of interceptor with a simple threadLocal managed within the SessionDelegate itself.
Thanks to Michale Duerig for simplified design!!

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1517503 13f79535-47bb-0310-9956-ffa450edef68