MEDIUM: htx/http-ana: No longer close connection on early HAProxy response

MEDIUM: htx/http-ana: No longer close connection on early HAProxy response

When a response was returned by HAProxy, a dedicated HTX flag was
set. Thanks to this flag, it was possible to add a "connection: close"
header to the response if the request was not fully received and to close
the connection. In the same way, when a redirect rule was applied,
keep-alive was forcefully disabled for unfinished requests.

All these mechanisms are now useless because the H1 mux is able to drain the
response. So HTX_FL_PROXY_RESP flag is removed and no special processing is
performed on HAProxy response when the request is unfinished.