mod_cache: Implement remove URL via a filter.

Remove entities from the cache when re-validation receives a 404 or other
content-no-longer-present error.

Suggested by: Paul Querna, Justin Erenkrantz
Submitted by: Rudiger Plum <ruediger.pluem vodafone.com>
Reviewed by: Justin Erenkrantz


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@231487 13f79535-47bb-0310-9956-ffa450edef68