Changes on ConfigurationNodePointer related to managing a NodeHandler.

These changes are required for adapting the NodeIterator implementations to
work with arbitrary node types supported by a NodeHandler.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1565789 13f79535-47bb-0310-9956-ffa450edef68
Adapted ConfigurationNodeIteratorBase.

This class is now able to provide a NodeHandler to its sub classes. Some
functionality related to the iteration has now to be implemented by sub
classes.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1565790 13f79535-47bb-0310-9956-ffa450edef68