Improve selection of the segment sequence number when restarting the
reception of a playlist after it was suspended due to being unneeded
(due to discard flags).

The current code assumes that each playlist contains matching data with
the same sequence number, while spec 3.4.3 specifically says that that
is not the case. Often subtitle playlists also have longer target
durations as well, causing the selection to be completely wrong.

Instead prefer using the playlist segment duration information for
non-live playlists, and other means if that is not possible.
Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>