mod_proxy: factorize mod_proxy_{connect,wstunnel} tunneling code in proxy_util.

mod_proxy: factorize mod_proxy_{connect,wstunnel} tunneling code in proxy_util.

This commit adds struct proxy_tunnel_rec that contains the fields needed for a
poll() loop through the filters chains, plus functions ap_proxy_tunnel_create()
and ap_proxy_tunnel_run() to respectively initialize a tunnel and (re)start it.

Proxy connect and wstunnel modules now make use of this new API to avoid
duplicating logic and code.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1869338 13f79535-47bb-0310-9956-ffa450edef68