mod_proxy: Improve tunneling loop.

mod_proxy: Improve tunneling loop.

Support half closed connections and pending data draining (for protocols like
rsync). PR 61616.

When reading on one side goes faster than writing on the other side, the output
filters chain may start buffering data and finally block, which will break
bidirectional tunneling for some protocols.

To avoid this, proxy_tunnel_run() now stops polling/reading until pending data
are drained, and recovers appropriately.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1869420 13f79535-47bb-0310-9956-ffa450edef68