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/requestlog/requestlog.go

@@ -182,6 +182,10 @@ type Line struct {
 	Reputation []string `json:"reputation,omitempty"`
 	// Tags are the request's tags, sorted.
 	Tags []string `json:"tags,omitempty"`
+	// RouteWatchTagsMissing, while SWW_ROUTEWATCH_URL is set, says why the
+	// request got no tags from routewatch: no_answer_yet, request_failed or
+	// no_route.
+	RouteWatchTagsMissing string `json:"routewatch_tags_missing,omitempty"`
 	// Offence is the offence the request was held as, OffenceLimit.
 	Offence string `json:"offence,omitempty"`
 	// BanExpires is when the ban the request made, or was refused under,