Better distribution of mirrors to crawl.
    
    The script mm2_get-highest-active-host-id used to return the highest
    ID of the active mirrors. This number was divided by the number of
    active crawlers and then each crawler got its share of mirrors to crawl.
    This did not take into account that more active mirrors are in the
    higher IDs as old mirror IDs are not re-used and thus one crawler was
    getting much more mirrors to crawl than another. The new script (which
    will be renamed) now divides the list correctly by returning exactly the
    fraction which each crawler should crawl.