Add a collection of aliases to MappedHost object to simplify enumeration.
This adds some cost to addHost/removeHost[Alias], but simplifies addContextVersion/removeContextVersion().
Require that addContextVersion(), removeContextVersion() methods were called on a real host. Ignore them on an alias.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1606680 13f79535-47bb-0310-9956-ffa450edef68