oggdemux: implement push mode seeking

oggdemux: implement push mode seeking

This patch implements seeking in push mode (eg, over the net)
in Ogg, using the double bisection method.
As a side effect, it also fixes duration determination of network
streams, by seeking to the end to check the actual duration.

Known issues:
- Getting an EOS while seeking stops the streaming task, I can't
  find a way to prevent this (eg, by issuing a seek in the event
  handler).
- Seeking twice in a VERY short succession with playbin2 fails
  for streams with subtitles, we end up pushing in a dataqueue
  which is flushing. Rare in normal use AFAICT.
- Seeking is slow on slow links - byte ranges guesses could be
  made better, decreasing the number of required requests
- If no granule position is found in the last 64 KB of a stream,
  duration will be left unknown (should be pretty rare)

https://bugzilla.gnome.org/show_bug.cgi?id=621897