dijkstra: fix heap ordering.

We were always ordering heap by distance, not score (which are different
if we are routing by cheapest, not shortest!).

This simplifies our callbacks, too.

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

devtools/route.c

@@ -52,7 +52,7 @@ static struct route **least_cost(struct gossmap *map,
 	tstart = time_mono();
 	dij = dijkstra(tmpctx, map, dst,
 		       sent, riskfactor, route_can_carry,
-		       route_path_cheaper, NULL);
+		       route_score_cheaper, NULL);
 	tstop = time_mono();
 
 	printf("# Time to find route: %"PRIu64" usec\n",