OAK-1958: Session.logout performance poor
Avoid Guava's performance hit caused by throwing a CancellationException on registering a listener on a completed feature by explicitly taking care of cancelling the task

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1609064 13f79535-47bb-0310-9956-ffa450edef68