Merge r684351, r686549 from trunk:

* Introduce environment variable proxy-initial-not-pooled to avoid reusing
  pooled connections if the client connection is an initial connection.
  This avoids the "proxy: error reading status line from remote server"
  error caused by the race condition that the backend server closed the
  connection after the connection check on our side and before our data
  reached the backend. Yes, this downgrades performance, especially with
  HTTP/1.0 clients. Hence it is configurable and off by default.

PR: 37770


* Add missing documentation for proxy-initial-not-pooled (r684351).



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@691230 13f79535-47bb-0310-9956-ffa450edef68