Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=56717
Fix duplicate registration of MapperListener during repeated starts of embedded Tomcat.
Properly unregister it when Tomcat stops.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1610562 13f79535-47bb-0310-9956-ffa450edef68