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

Access to base field in Unconnected does not guaranteed it is up-to-date, use base() instead.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1558761 13f79535-47bb-0310-9956-ffa450edef68