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/proxy.c b/src/proxy.c
index 85d7562..9b23db0 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -960,8 +960,8 @@
 	 * have to re-adjust the desired keep-alive/close mode to accommodate
 	 * both the frontend's and the backend's modes.
 	 */
-	if (s->fe->mode == PR_MODE_HTTP && be->mode == PR_MODE_HTTP &&
-	    ((s->fe->options & PR_O_HTTP_MODE) != (be->options & PR_O_HTTP_MODE)))
+	if (strm_sess(s)->fe->mode == PR_MODE_HTTP && be->mode == PR_MODE_HTTP &&
+	    ((strm_sess(s)->fe->options & PR_O_HTTP_MODE) != (be->options & PR_O_HTTP_MODE)))
 		http_adjust_conn_mode(s, &s->txn, &s->txn.req);
 
 	/* If an LB algorithm needs to access some pre-parsed body contents,