MEDIUM: applet: make the applets only use si_applet_{cant|want|stop}_{get|put}

The applets don't fiddle with SI_FL_WAIT_ROOM anymore, instead they indicate
what they want, possibly that they failed (eg: WAIT_ROOM), and it's done() /
update() which finally updates the WAIT_* flags according to the channels'
and stream interface's states. This solves the issue of the pauses during a
"show sess" without creating busy loops.
diff --git a/src/peers.c b/src/peers.c
index 5b6d817..26b3e6f 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -1046,7 +1046,7 @@
 	si_oc(si)->flags |= CF_READ_DONTWAIT;
 	return;
 full:
-	si->flags |= SI_FL_WAIT_ROOM;
+	si_applet_cant_put(si);
 	goto out;
 }