ModelTransaction now uses a visitor when removing nodes from the parent mapping.

The iteration loop for traversing the nodes hierarchy has been replaced by a
call to NodeTreeWalker.walkBFS().

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1570347 13f79535-47bb-0310-9956-ffa450edef68