Merge of r1750392,r1750412,r1750416,r1750474,r1750494,r1750508 from trunk:

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



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1766372 13f79535-47bb-0310-9956-ffa450edef68