BUG/MEDIUM: http-client: Drain the request if an early response is received

BUG/MEDIUM: http-client: Drain the request if an early response is received

When a large request is sent, it is possible to have a response before the
end of the request. It is valid from HTTP perspective but it is an issue
with the current design of the http-client. Indded, the request and the
response are handled sequentially. So the response will be blocked, waiting
for the end of the request. Most of time, it is not an issue, except when
the request transfer is blocked. In that case, the applet is blocked.

With the current API, it is not possible to handle early response and
continue the request transfer. So, this case cannot be handle. In that case,
it seems reasonnable to drain the request if a response is received. This
way, the request transfer, from the caller point of view, is never blocked
and the response can be properly processed.

To do so, the action flag HTTPCLIENT_FA_DRAIN_REQ is added to the
http-client. When it is set, the request payload is just dropped. In that
case, we take care to not report the end of input to properly report the
request was truncated, especially in logs.

It is only an issue with large POSTs, when the payload is streamed.

This patch must be backported as far as 2.6.

(cherry picked from commit 25b0625d5c27a209c38dcac6a81c58495e5360af)
[ad: context adjustement due to missing HTTPCLIENT_O_RES_HTX in 3.2]
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>
(cherry picked from commit eadb777452f8470ddaeb0ffdb59c130ce1621823)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit fff555a50b229a32341931e7928d19fc7b023789)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>