Mapper improvements
- Mark MappedContext.versions field as volatile, because it is updated when a Context is started or stopped.
- Mark MapElement fields as final and add them to constructors.
- Change super class of MappedContext from MapElement<Context> to MapElement<Void>. It does not wrap any real object, its "object" field is always null.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1604320 13f79535-47bb-0310-9956-ffa450edef68