mod_proxy_http: handle async tunneling of Upgrade(d) protocols.

mod_proxy_http: handle async tunneling of Upgrade(d) protocols.

When supported by the MPM (i.e. "event"), provide async callbacks and let
them be scheduled by ap_mpm_register_poll_callback_timeout(), while the
handler returns SUSPENDED.

The new ProxyAsyncDelay directive (if positive) enables async handling,
while ProxyAsyncIdleTimeout determines the timeout applied on both ends
while tunneling.

Github: closes #126

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1879419 13f79535-47bb-0310-9956-ffa450edef68