Ensure that the root node of the newly created CombinedConfiguration is initialized when it is returned from the builder.

This changes makes it possible to use read-only combined configurations
with a NoOpSynchronizer because no further initialization is needed.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1488577 13f79535-47bb-0310-9956-ffa450edef68
Added Initializable interface.

This interface is used by configuration builder implementations to find out
whether a configuration instance needs special initialization.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1488578 13f79535-47bb-0310-9956-ffa450edef68