mod_cache: try to use the key of a possible open but stale cache entry

if we have one in cache_try_lock(). PR 50317

Submitted by: Ruediger Pluem


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1597533 13f79535-47bb-0310-9956-ffa450edef68