Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=56653
Fix concurrency issue with lists of contexts in Mapper when stopping Contexts.

The ContextList fields are made final.
Instead of sharing the same ContextList object instance among Hosts,
the ContextList is made an immutable object and is updated via updateContextList() method.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1608993 13f79535-47bb-0310-9956-ffa450edef68