en for each priority level, a running sum is calculated
        over the sorted list of records for that priority. Then a random value
        between 0 and the final sum is compared to each record in order. The
        first record that is greater than or equal to that random value is
        chosen and removed from the list of candidates for this round.

        @return: A tuple of target hostname and port from the chosen DNS SRV
            record.
        @rtype: L{tuple} of native L{str} and L{int}
        Nc