choose: Use a one-pass algorithm to select tracks.

choose: Use a one-pass algorithm to select tracks.

The previous algorithm used one pass over the track database to compute
weights and build a list, and a second pass to pick the right track from
the list after having actually chosen a random number,

This change throws out a lot of the old code involved with constructing
and walking the list by using a one-pass algorithm instead.  (The
algorithm provably preserves the probability distribution: see
commentary.)