Added selectAndTrackNodes() method to InMemoryNodeModel.

This method is needed for the new implementation of configurationsAt(). It
allows tracking multiple nodes selected by a key at once.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1580041 13f79535-47bb-0310-9956-ffa450edef68
Added trackChildNodes() method to InMemoryNodeModel.

This method allows tracking of all children of a selected node. It will be used
for the new implementation of childConfigurationsAt().

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1580042 13f79535-47bb-0310-9956-ffa450edef68