paymod: Keep a copy of the serialized bolt11 in the root payment

We frequently query by the bolt11 string, so keeping it around saves us from
having to parse the query or serialize the parsed one.

plugins/pay.c

@@ -1780,6 +1780,7 @@ static struct command_result *json_paymod(struct command *cmd,
 				? tal_dup(p, struct secret, b11->payment_secret)
 				: NULL;
 	p->invoice = tal_steal(p, b11);
+	p->bolt11 = tal_steal(p, b11str);
 	payment_start(p);
 	list_add_tail(&payments, &p->list);