TreeData now implements the NodeHandler interface.

The current solution that InMemoryNodeModel implements NodeHandler is
problematic because of the implementation of the getParent() method: there can
be race conditions if the model was updated and now uses a different TreeData
instance. Therefore, all this functionality has to be kept in a single place:
in TreeData.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1575759 13f79535-47bb-0310-9956-ffa450edef68