routing: Returning channel_id to getroute requests

The new onion uses the `channel_id` instead of the `node_id` of the
next hop to identify where to forward the payment. So we return the
exact channel chosen by the routing algo, to avoid having to look it
up again later.

daemon/routing.h

@@ -90,6 +90,7 @@ struct routing_state {
 };
 
 struct route_hop {
+	struct short_channel_id channel_id;
 	struct pubkey nodeid;
 	u32 amount;
 	u32 delay;