OAK-1143: [scala] Repository init throws "illegal cyclic reference involving class ChangeDispatcher"
Make ChangeDispatcher implement Observer and replace beforeCommit, localCommit and afterCommit method with Observer.contentChanged

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1540983 13f79535-47bb-0310-9956-ffa450edef68