Reworked update and event handling for sub configurations.

The complicated mechanism to detect updates in sub configurations, propagate
them, and validate root nodes is no longer needed. This is all handled by
tracked nodes. Now a simple change listener is registered at connected sub
configurations which sends events of type EVENT_SUBNODE_CHANGED.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1578588 13f79535-47bb-0310-9956-ffa450edef68
Catch exceptions when reevaluate the keys of tracked nodes.

It is possible that the evaluation of a key causes an exception to be thrown,
e.g. if the expression engine has changed in the meantime. In this case, the
node becomes detached.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1578589 13f79535-47bb-0310-9956-ffa450edef68