Added a replaceTrackedNode() method to InMemoryNodeModel.

This method is useful when working on sub trees of the model. It is then
possible to detach a whole sub tree and to replace its root node.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1578574 13f79535-47bb-0310-9956-ffa450edef68