Arrange packet arr by decreasing shot-origin-time

The trilateration strategy involves trying to minimize errors in situations
where packets could belong to one shot or another.  To handle this, packets
are examined in order of decreasing shot-origin-time (the time the shot
was fired, not detected).  This lets the most recent shots get triangulated
first, rather than potentially letting older packets sneak in and snatch up
packets that belong to a more recent gunshot.