lib/connections: Fix and optimize registry (#7996)

Registry.Get used a full sort to get the minimum of a list, and the sort
was broken because util.AddressUnspecifiedLess assumed it could find out
whether an address is IPv4 or IPv6 from its Network method. However,
net.(TCP|UDP)Addr.Network always returns "tcp"/"udp".

lib/dialer/public.go

@@ -110,7 +110,9 @@ func DialContextReusePort(ctx context.Context, network, addr string) (net.Conn,
 		return DialContext(ctx, network, addr)
 	}
 
-	localAddrInterface := registry.Get(network, tcpAddrLess)
+	localAddrInterface := registry.Get(network, func(addr interface{}) bool {
+		return addr.(*net.TCPAddr).IP.IsUnspecified()
+	})
 	if localAddrInterface == nil {
 		// Nothing listening, nothing to reuse.
 		return DialContext(ctx, network, addr)