Read the drop counter atomically and set the WHOIS fetcher before serving (closes #115)

The read loop added to a handler's drop count atomically while the
statistics endpoint and the metrics log read it as a plain field, a data
race whenever messages were dropped. The count is now an atomic.Uint64,
read with Load. A test reads the handler statistics while the read loop
drops messages for a handler that never finishes one.

Run created the WHOIS fetcher and gave it to the HTTP server after the
server had started, while request handlers could read it. The fetcher is
now set before the server starts.

Model: opus-5-5

internal/server/server.go

@@ -139,7 +139,8 @@ func (s *Server) Stop(ctx context.Context) error {
 	return nil
 }
 
-// SetASNFetcher sets the ASN WHOIS fetcher for on-demand lookups.
+// SetASNFetcher sets the ASN WHOIS fetcher for on-demand lookups. Call it
+// before Start: the handlers read the fetcher without a lock.
 func (s *Server) SetASNFetcher(fetcher ASNFetcher) {
 	s.asnFetcher = fetcher
 }