Reworked DynamicCombinedConfiguration regarding thread-safety.

The class now uses its Synchronizer to protect its internal state against
concurrent access. All tests could be enabled again (after configuring a
fully functional Synchronizer).

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1488570 13f79535-47bb-0310-9956-ffa450edef68
Added a chapter about thread-safety to the user's guide.

This is an initial version which may need some updates later.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1488571 13f79535-47bb-0310-9956-ffa450edef68