Change format result data to slice

[sitnikovv]

The format of the returned data has been changed from a map to a slice, since in a slice the data is already arranged in the required order.
Made as v3 since there is no backward compatibility.
The OrderSRV function was made public in order to make it possible in user functions to optionally sort data already received through net.LookupSRV and there was no need to go through this procedure again (DNS queries can be very long).
For example:

func ServiceDiscovery(service, proto, host string, sortWithWeight bool) ([]string, error) {
	_, addrs, err := net.LookupSRV(service, proto, host)
	if err != nil {
		return nil, err
	}
	if sortWithWeight {
		_, addrs = OrderSRV(addrs)
	}

	result := make([]string, len(addrs))
	for i, srv := range addrs {
		result[i] = strings.TrimRight(srv.Target, ".") + ":" + strconv.Itoa(int(srv.Port))
	}

	return result, nil
}