Fixed a dead lock problem with DynamicCombinedConfiguration.

There was a problem with read operations when a new child configuration had to
be created. This requires a write lock which caused the thread to hang. The
new solution determines the current configuration at the very begin of an
operation.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1488576 13f79535-47bb-0310-9956-ffa450edef68