HierarchicalConfiguration now extends NodeModelSupport.

The new method was implemented in AbstractHierarchicalConfiguration. Access to
the NodeModel is guarded by the configuration's Synchronizer.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1587024 13f79535-47bb-0310-9956-ffa450edef68
Added InMemoryNodeModelSupport.

This is a special extension of NodeModelSupport for models of type
InMemoryNodeModel. It is needed by special model implementations (mainly
TrackedNodeModel) which have to access an InMemoryNodeModel object.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1587025 13f79535-47bb-0310-9956-ffa450edef68