BUG/MEDIUM: peers: recent applet changes broke peers updates scheduling

Since appctx are scheduled out of streams, it's pointless to wake up
the task managing the stream to push updates, they won't be seen. In
fact unit tests work because silent sessions are restarted after 5s of
idle and the exchange is correctly scheduled during startup!

So we need to notify the appctx instead. For this we add a pointer to
the appctx in the peer session.

No backport is needed of course.
diff --git a/include/types/peers.h b/include/types/peers.h
index 8f8eefa..8b7712a 100644
--- a/include/types/peers.h
+++ b/include/types/peers.h
@@ -39,6 +39,7 @@
 	struct shared_table *table;   /* shared table */
 	struct peer *peer;	      /* current peer */
 	struct stream *stream;        /* current transport stream */
+	struct appctx *appctx;        /* the appctx running it */
 	unsigned int flags; 	      /* peer session flags */
 	unsigned int statuscode;      /* current/last session status code */
 	unsigned int update;	      /* current peer acked update */