BUG/MINOR: http-ana: Don't wait to send 1xx responses generated by HAProxy

BUG/MINOR: http-ana: Don't wait to send 1xx responses generated by HAProxy

When an informational response (1xx) is returned by HAProxy, we must be sure to
send it ASAP. To do so, CF_SEND_DONTWAIT flag must be set on the response
channel to instruct the stream-interface to not set the CO_SFL_MSG_MORE flag on
the transport layer. Otherwise the response delivery may be delayed, because of
the commit 8945bb6c0 ("BUG/MEDIUM: stream-int: fix loss of CO_SFL_MSG_MORE flag
in forwarding").

This patch may be backported as far as 1.9, for HTX part only. But this part has
changed in the 2.2, so it may be a bit tricky. Note it does not fix any known
bug on previous versions because the CO_SFL_MSG_MORE flag is ignored by the h1
mux.