MEDIUM: stream: move the frontend's pointer to the session

Just like for the listener, the frontend is session-wide so let's move
it to the session. There are a lot of places which were changed but the
changes are minimal in fact.
diff --git a/src/hlua.c b/src/hlua.c
index 1a0b920..2934054 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -2139,8 +2139,8 @@
 	socket->s->flags = 0;
 
 	/* Assign the configured proxy to the new stream. */
+	socket->s->sess->fe = &socket_proxy;
 	socket->s->be = &socket_proxy;
-	socket->s->fe = &socket_proxy;
 
 	/* XXX: Set namy variables */
 	socket->s->store_count = 0;