mod_proxy_http: don't connect or reuse backend before prefetching request body.

mod_proxy_http: don't connect or reuse backend before prefetching request body.

The goal is to minimize the delay between this connection is considered alive
and the first bytes sent (should the client's link be slow or some input filter
retain the data).
This is a best effort to prevent the backend from closing (from under us) what
it thinks is an idle connection, hence to reduce to the minimum the unavoidable
local ap_proxy_is_socket_connected() vs remote keepalive race condition.
PR 56541.

Also, allow the new subprocess_env variable "proxy-flushall" to prevent any
buffering of the request body before it is forwarded to the backend.
When set, the prefetch is still done (although non-blocking), so we can still
determine Content-Length vs chunked vs spooled (depending on data available
with the header or while reading it), and then all brigades are flushed when
passed to the backend.
PR 37920.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1656259 13f79535-47bb-0310-9956-ffa450edef68