Ensure season_num is set properly on allocation

[markspolakovs]

We've had a few reports of two seasons on the same show both showing up as "season 1", which goes away once the cache is bopped. Here's my hunch as to what's happening:

1. when MyRadio_Season::getInstance is called, before allocate() is called, it computes season_num through SQL:

(
    SELECT COUNT(*) FROM schedule.show_season
    WHERE show_id=(SELECT show_id FROM schedule.show_season WHERE show_season_id=$1)
    AND show_season_id<=$1
    AND show_season_id IN (SELECT show_season_id FROM schedule.show_season_timeslot)
) AS season_num

At this point, the unallocated season has a season_num of the last allocated one.
2. MyRadio_Season->allocate() is called, which schedules timeslots, but does not update season_num
3. The object is written back to the cache, with the incorrect season_num.

The fix is likely to re-compute season_num in allocate().

For #1054.