Made method clear() final in AbstractConfiguration.

There is now a new protected clearDirect() method which performs the actual
work. This is done to prevent subclasses from breaking basic functionality
like event handling or synchronization.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1479367 13f79535-47bb-0310-9956-ffa450edef68