InMemoryNodeModel now provides support for tracking nodes.

Nodes can be tracked by specifying a corresponding NodeSelector. After updates
of the model, the node instances associated with this key are still available.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1575756 13f79535-47bb-0310-9956-ffa450edef68