Renamed TreeData.getRoot() to getRootNode().

This is in sync with other classes.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1575761 13f79535-47bb-0310-9956-ffa450edef68
Added method getRootNode() to NodeHandler interface.

A node handler is always associated with a specific nodes structure. If the
handler and the root node are queried from different objects, there is a
potential race condition that the nodes structure has been updated in the mean
time.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1575762 13f79535-47bb-0310-9956-ffa450edef68