Added TrackedNodeModel class.

This is a special NodeModel implementation which operates on a tracked node.
It will be used for the new implementation of sub node configurations.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1578576 13f79535-47bb-0310-9956-ffa450edef68
Added functionality to NodeStructureHelper to create flexible table trees.

It is now possible to create the tree structure with database tables and their
fields in a flexible manner.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1578578 13f79535-47bb-0310-9956-ffa450edef68