Added synchronization to read methods of BaseHierarchicalConfiguration.

Some methods have been made final to prevent subclasses from breaking
synchronization. Corresponding protected methods with the suffix 'Internal'
were added.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1479379 13f79535-47bb-0310-9956-ffa450edef68