MINOR: stream: Add pointer to front/back conn-streams into stream struct

frontend and backend conn-streams are now directly accesible from the
stream. This way, and with some other changes, it will be possible to remove
the stream-interfaces from the stream structure.
diff --git a/src/backend.c b/src/backend.c
index 8367a8e..34b53af 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1496,7 +1496,7 @@
 
 			if (avail >= 1) {
 				si_attach_conn(&s->si[1], srv_conn);
-				if (srv_conn->mux->attach(srv_conn, s->si[1].cs, s->sess) == -1) {
+				if (srv_conn->mux->attach(srv_conn, s->csb, s->sess) == -1) {
 					si_reset_endpoint(&s->si[1]);
 					srv_conn = NULL;
 				}
@@ -1679,7 +1679,7 @@
 	if (init_mux) {
 		const struct mux_ops *alt_mux =
 		  likely(!(s->flags & SF_WEBSOCKET)) ? NULL : srv_get_ws_proto(srv);
-		if (conn_install_mux_be(srv_conn, s->si[1].cs, s->sess, alt_mux) < 0) {
+		if (conn_install_mux_be(srv_conn, s->csb, s->sess, alt_mux) < 0) {
 			conn_full_close(srv_conn);
 			return SF_ERR_INTERNAL;
 		}
@@ -1741,7 +1741,7 @@
 	 * sockets, socket pairs, and occasionally TCP connections on the
 	 * loopback on a heavily loaded system.
 	 */
-	if ((srv_conn->flags & CO_FL_ERROR || (s->si[1].cs)->flags & CS_FL_ERROR))
+	if ((srv_conn->flags & CO_FL_ERROR || s->csb->flags & CS_FL_ERROR))
 		s->si[1].flags |= SI_FL_ERR;
 
 	/* If we had early data, and the handshake ended, then
@@ -1750,7 +1750,7 @@
 	 * the handshake.
 	 */
 	if (!(srv_conn->flags & (CO_FL_WAIT_XPRT | CO_FL_EARLY_SSL_HS)))
-		(s->si[1].cs)->flags &= ~CS_FL_WAIT_FOR_HS;
+		s->csb->flags &= ~CS_FL_WAIT_FOR_HS;
 
 	if (!si_state_in(s->si[1].state, SI_SB_EST|SI_SB_DIS|SI_SB_CLO) &&
 	    (srv_conn->flags & CO_FL_WAIT_XPRT) == 0) {
@@ -1767,7 +1767,7 @@
 	 *       wake callback. Otherwise si_cs_recv()/si_cs_send() already take
 	 *       care of it.
 	 */
-	if (((s->si[1].cs)->flags & CS_FL_EOI) && !(si_ic(&s->si[1])->flags & CF_EOI))
+	if ((s->csb->flags & CS_FL_EOI) && !(si_ic(&s->si[1])->flags & CF_EOI))
 		si_ic(&s->si[1])->flags |= (CF_EOI|CF_READ_PARTIAL);
 
 	/* catch all sync connect while the mux is not already installed */
@@ -2076,7 +2076,7 @@
  */
 void back_handle_st_req(struct stream *s)
 {
-	struct stream_interface *si = &s->si[1];
+	struct stream_interface *si = cs_si(s->csb);
 
 	if (si->state != SI_ST_REQ)
 		return;
@@ -2085,7 +2085,7 @@
 
 	if (unlikely(obj_type(s->target) == OBJ_TYPE_APPLET)) {
 		/* the applet directly goes to the EST state */
-		struct appctx *appctx = cs_appctx(si->cs);
+		struct appctx *appctx = cs_appctx(s->csb);
 
 		if (!appctx || appctx->applet != __objt_applet(s->target))
 			appctx = si_register_handler(si, objt_applet(s->target));
@@ -2211,7 +2211,7 @@
  */
 void back_handle_st_cer(struct stream *s)
 {
-	struct stream_interface *si = &s->si[1];
+	struct stream_interface *si = cs_si(s->csb);
 
 	DBG_TRACE_ENTER(STRM_EV_STRM_PROC|STRM_EV_SI_ST, s);
 
@@ -2220,7 +2220,7 @@
 
 	/* we probably have to release last stream from the server */
 	if (objt_server(s->target)) {
-		struct connection *conn = cs_conn(si->cs);
+		struct connection *conn = cs_conn(s->csb);
 
 		health_adjust(__objt_server(s->target), HANA_STATUS_L4_ERR);