FileHandler now takes care for synchronization.

If the associated FileBased object implements the SynchronizerSupport
interface, the object is locked during load and save operations.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1482086 13f79535-47bb-0310-9956-ffa450edef68
Some special methods of XMLConfiguration now call the Synchronizer correctly.
Updated Javadoc regarding thread-safety.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1482087 13f79535-47bb-0310-9956-ffa450edef68