mod_proxy_http: forward 100-continue, and minimize race conditions when

reusing backend connections. PR 60330.

+1: ylavic, icing, jim
ylavic: plus http://svn.apache.org/r1856036 (opt-out)
2.4.x patch: http://people.apache.org/~ylavic/patches/httpd-2.4.x-forward_100_continue-v6.patch
+1: ylavic, jim, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1860166 13f79535-47bb-0310-9956-ffa450edef68