OAK-659: Move purge logic for transient changes below the NodeBuilder interface

No more need for copy() and move() to be on NodeStoreBranch, so push them
down to AbstractNodeStoreBranch for use by backends that still need them.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1547244 13f79535-47bb-0310-9956-ffa450edef68