BaseHierarchicalConfiguration now implements the Initializable interface.

In initialize() internal data structures for managing SubnodeConfigurations are
initialized. So this initialization does not have to be performed lazily,
which makes concurrent access to SubnodeConfigurations possible without
having to use a fully functional Synchronizer.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1488580 13f79535-47bb-0310-9956-ffa450edef68