Initial integration of Synchronizer into AbstractConfiguration.

This is not yet complete. Currently, methods of AbstractConfiguration calling
the Synchronizer can be overridden by subclasses. This breaks proper
synchronization.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1479366 13f79535-47bb-0310-9956-ffa450edef68