Change 90 days update: update only 10 oldest stale shows per session

Distribute the updates of real 'Ended' shows in smaller portions,
prevent hitting thetvdb with all stale 'Ended' in one update session.

+ moved should_update to TVShow object
+ use same update_date variable during session
+ changed some variable names

Signed-off-by: Patrick Vos <itofzo@gmail.com>