backport some consistency with trunks mod_cache:

backport some consistency with trunks mod_cache:

   * mod_cache, mod_disk_cache: Try to use the key of a possible open but
     stale cache entry if there is one. This fixes problem when two different
     cache locks have been created for single stale cache entry leading to two
     requests sent to backend.
     PR 50317

   * Remove useless apr_file_remove() before renaming the cache entry in
     mod_disk_cache. This fixes small time-frame during which stale cache
     entry can be seen as not-cached.
     PR 50317

Subitted By: rpluem, jkaluza, ylavic
Reviewed By: ylavic, rpluem, jkaluza

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1608838 13f79535-47bb-0310-9956-ffa450edef68