improve level switching algorithm : when selecting a new level, ensure that there would be sufficient time to download 2 fragments from this new level.
fix 404 issue on live playlists when window in cache and window in server are not synchronized. the IO Error is now reported if it still happens after X-TARGET-DURATION
fix adaptive playlist switch issue: add a small tolerance for PTS value (to avoid reporting a PTS out of bound to Buffer.as)