mod_proxy: don't recyle backend announced "Connection: close" connections

to avoid reusing it should the close be effective after some new request
is ready to be sent.

Backports: r1678763, r1703807, r1703813, r1678763
Submitted by: ylavic
Reviewed by: rpluem, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1750836 13f79535-47bb-0310-9956-ffa450edef68