mod_cache: Introduce the thundering herd lock, a mechanism to keep

the flood of requests at bay that strike a backend webserver as
a cached entity goes stale.
+1: minfrin, jim, pgollucci


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@917013 13f79535-47bb-0310-9956-ffa450edef68