mod_cache: Add the cache_status hook to register the final cache

decision hit/miss/revalidate. Add optional support for an X-Cache
and/or an X-Cache-Detail header to add the cache status to the
response. PR48241


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1001639 13f79535-47bb-0310-9956-ffa450edef68