* mod_cache: Don't remove stale cache entries that cannot be conditionally

     revalidated. This prevents the thundring herd protection from serving
     stale during a revalidation. Reverts most of r572626 which is also gone
     from later branches.   PR 50317.

Submitted By: covener, jkaluza, rpluem
Reviewed By: covener, jkaluza, ylavic


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1608302 13f79535-47bb-0310-9956-ffa450edef68