paymod: Let the payment know the local ID

This is necessary for the routehint modifier.

plugins/libplugin-pay.h

@@ -150,6 +150,7 @@ struct payment {
 	struct command *cmd;
 	struct plugin *plugin;
 	struct list_node list;
+	struct node_id *local_id;
 
 	const char *json_buffer;
 	const jsmntok_t *json_toks;

plugins/pay.c

@@ -1887,6 +1887,7 @@ static struct command_result *json_paymod(struct command *cmd,
 				    "Invalid bolt11:"
 				    " sets feature var_onion with no secret");
 
+	p->local_id = &my_id;
 	p->json_buffer = tal_steal(p, buf);
 	p->json_toks = params;
 	p->destination = p->getroute_destination = &b11->receiver_id;