connectd: hold peer until we're interested.

Either because lightningd tells us it wants to talk, or because the peer
says something about a channel.

We also introduce a behavior change: we disconnect after a failed open.
We might want to modify this later, but we it's a side-effect of openingd
not holding onto idle connections.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

connectd/connectd.h

@@ -63,6 +63,9 @@ struct peer {
 	 * it's done). */
 	bool ready_to_die;
 
+	/* Has this ever been active?  (i.e. ever had a subd attached?) */
+	bool active;
+
 	/* When socket has Nagle overridden */
 	bool urgent;