Merge r1568404 from trunk:

mod_proxy_http: don't recycle backend connections known to be closed (eg. EOS by close).
This saves a useless ap_is_socket_connected() call when reused.

Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1573357 13f79535-47bb-0310-9956-ffa450edef68