Added a final getProperty() method to AbstractConfiguration.

This method is responsible for synchronization handling. There is now a new
protected getPropertyInternal() method which has to be defined by concrete
subclasses.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1479371 13f79535-47bb-0310-9956-ffa450edef68