Add asn= and prefix= tags from a routewatch instance (closes #204)

SWW_ROUTEWATCH_URL names the instance. The new heuristic routewatch_tags
asks it in the background about each client and tags the request. Answers
are kept in memory for an hour. A request without tags gives the reason in
routewatch_tags_missing, and it is never delayed or refused.

Model: opus-5-5

internal/discriminator/heuristics.go

@@ -44,6 +44,7 @@ const (
 func beforeLookup() []heuristic {
 	return []heuristic{
 		tagLists{},
+		routeWatchTags{},
 		denyNets{},
 		activeBan{},
 	}