dashdemux: Improve live stream manifest update repositioning

dashdemux: Improve live stream manifest update repositioning

When updating the manifest the timestamps on it might have changed a little
due to rounding and timescale conversions. If the change makes the timestamp
of the current segment to go up it makes dashdemux reposition to the previous
one causing one extra unnecessary download.

So when repositioning add an extra 10 microseconds to cover for that rounding
issues and increase the chance of falling in the same segment.

Additionally, also improve the time used when the client is already after the
last segment. Instead of using the last segment starting timestamp use the
final timestamp to make it reposition to the next one and not to the one that
has already been downloaded.