If the MPM supports serf in its core, use this to drive the proxied request

completely async from the original thread.

Right now it uses a 100ms timer to determine when the request is actaully complete,
but I think this can be removed in the long run with a better mechanism.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@759414 13f79535-47bb-0310-9956-ffa450edef68