Reworked CombinedConfiguration regarding thread-safety.

The Synchronizer object is now exclusively used to protect an instance against
concurrent access. This is a change of behavior because previously there was
always some kind of synchronization; now this is configurable. Therefore,
currently some tests for multi-threaded access are failing and have been
disabled.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1485392 13f79535-47bb-0310-9956-ffa450edef68