[CONFIGURATION-572] Fixed a possible memory leak.

When a CombinedConfiguration is cleared it is now ensured that change listener
registrations are removed for all child configurations.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1590417 13f79535-47bb-0310-9956-ffa450edef68
[CONFIGURATION-572] Updated changes.xml.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1590418 13f79535-47bb-0310-9956-ffa450edef68