Draft: selection: balance the choice of IPv4/IPv6 servers when RTT cache is clear

On clear RTT and in the presence of miscunfigured IP protocol version (e.g. net.ipv6 enabled but IPv6 doesn't work), resolver might waste a lot of time and packets trying to query nameservers using the broken protocol.

We choose the protocol first and if there is some candidate server with unknown RTT and using the chosen protocol, we swap it to the front and it is used for the transport.

If there is none, tough luck, we still want to try all the candidates before going back trying some again.

lib/selection.c

@@ -331,7 +331,16 @@ static void shuffle_choices(struct choice choices[], int choices_len)
 static void balance_ip_versions(struct choice choices[], int choices_len)
 {
 	size_t versions[2] = {sizeof(struct in_addr), sizeof(struct in6_addr)};
-	size_t version_len = versions[kr_rand_coin(1,2)];
+	static int chosen_version = 0;
+	if (chosen_version == 0) {
+		chosen_version = kr_rand_coin(1,2)+1;
+	} else if (chosen_version == 1) {
+		chosen_version = 2;
+	} else if (chosen_version == 2) {
+		chosen_version = 1;
+	}
+
+	int version_len = versions[chosen_version - 1];
 
 	for (int i = 0; i < choices_len; i++) {
 		if (!no_rtt_info(choices[i].address_state->rtt_state)) {