LOG4J2-406: Unregister all log4j JMX MBeans when the LoggerContext is stopped to allow web application classes to be GC-ed on undeploy.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1557654 13f79535-47bb-0310-9956-ffa450edef68