Backport r808723, which was applied to trunk but not added to the backport

proposal below. Fixes a compiler warning on v2.2 reported by Rainer Jung.
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.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@917177 13f79535-47bb-0310-9956-ffa450edef68