Reworked PropertiesConfiguration regarding thread-safety.

Some special methods of PropertiesConfiguration now call the Synchronizer
correctly. Removed synchronization from properties set via the builder.
Updated Javadoc regarding thread-safety.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1483059 13f79535-47bb-0310-9956-ffa450edef68
Updated Javadocs related to thread-safety.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1483060 13f79535-47bb-0310-9956-ffa450edef68