Only fetch torrent files actually needed (speedup)

- Previously during a backlog search all season torrents for a specific
  season would be fetched regardless of quality. So if there were
  10 season torrents for season 1 all 10 torrent files would be fetched,
  but of course, in the end only one is sent to the torrent client.
  This commit delays the fetching of the torrent file until we know
  for sure that we need it (right before sending it to the torrent client).
- The speedup might be over 10 folds for some backlog searches.
- Magnet link torrent providers (like thepiratebay) are not affected
  by this issue.