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>

lightningd/channel.h

@@ -67,6 +67,9 @@ struct open_attempt {
 	struct command *cmd;
 	struct amount_sat funding;
 	const u8 *our_upfront_shutdown_script;
+
+	/* First msg to send to dualopend (to make it create channel) */
+	const u8 *open_msg;
 };
 
 struct channel {