[BUG] connect timeout is in the stream interface, not the buffer

The connect timeout was not properly detected due to the fact that
it was not correctly initialized. It must be set as the stream interface
timeout, not the buffer's write timeout.

src/backend.c

@@ -1840,7 +1840,7 @@ int connect_server(struct session *s)
 			s->be->lbprm.server_take_conn(s->srv);
 	}
 
-	s->req->wex = tick_add_ifset(now_ms, s->be->timeout.connect);
+	s->req->cons->exp = tick_add_ifset(now_ms, s->be->timeout.connect);
 	return SN_ERR_NONE;  /* connection is OK */
 }