mod_proxy_{http,ajp,fcgi}}: don't reuse backend connections with data available

mod_proxy_{http,ajp,fcgi}}: don't reuse backend connections with data available
before the request is sent.  PR 57832.

ap_proxy_check_backend() can be used before ap_proxy_connect_backend() to try
to read available data (including from the filters), and is called by
ap_proxy_connect_backend() to check the socket state only (as before, still
relevant after ap_proxy_check_backend() due to filter data which may not have
triggered a real socket operation).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1750392 13f79535-47bb-0310-9956-ffa450edef68