MINOR: stream-int: split si_update() into si_update_rx() and si_update_tx()

We should not update the two directions at once, in fact we should update
the Rx path after recv() and the Tx path after send(). Let's start by
splitting the update function in two for this.
diff --git a/include/proto/stream_interface.h b/include/proto/stream_interface.h
index 46df1bd..7202443 100644
--- a/include/proto/stream_interface.h
+++ b/include/proto/stream_interface.h
@@ -45,7 +45,8 @@
 int conn_si_send_proxy(struct connection *conn, unsigned int flag);
 struct appctx *si_register_handler(struct stream_interface *si, struct applet *app);
 void si_applet_wake_cb(struct stream_interface *si);
-void si_update(struct stream_interface *si);
+void si_update_rx(struct stream_interface *si);
+void si_update_tx(struct stream_interface *si);
 int si_cs_recv(struct conn_stream *cs);
 struct task *si_cs_io_cb(struct task *t, void *ctx, unsigned short state);
 void si_update_both(struct stream_interface *si_f, struct stream_interface *si_b);
@@ -529,6 +530,13 @@
 	return ret;
 }
 
+/* Combines both si_update_rx() and si_update_tx() at once */
+static inline void si_update(struct stream_interface *si)
+{
+	si_update_rx(si);
+	si_update_tx(si);
+}
+
 /* Returns info about the conn_stream <cs>, if not NULL. It call the mux layer's
  * get_cs_info() function, if it exists. On success, it returns a cs_info
  * structure. Otherwise, on error, if the mux does not implement get_cs_info()