lightningd: make "is peer connected" a tristate.

First, connectd tells us the peer has connected, and we call the connected hook,
and if it says it's fine, we are actually connected and we fire off notifications.

Of course, we could be disconnected while in the connected hook, and that would
mean we tell people about a connection which is no longer current.

Make this clear with a tristate: if we're not marked disconnected by
the time the hooks finish, we're good.  It also gives us a cleaner
"connect" command return when we connected but disconnected before
processing.

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

lightningd/connect_control.h

@@ -20,6 +20,7 @@ void try_reconnect(const tal_t *ctx,
 void connect_succeeded(struct lightningd *ld, const struct peer *peer,
 		       bool incoming,
 		       const struct wireaddr_internal *addr);
+void connect_failed_disconnect(struct lightningd *ld, const struct node_id *id);
 
 /* Disconnect a peer (if no subds want to talk any more) */
 void maybe_disconnect_peer(struct lightningd *ld, struct peer *peer);