Feature/dash handler optimizations

[dsilhavy]

This PR aims to solve parts of #3713. It includes a new logic in the DashHandler to decide whether all segments of a period have been loaded:

• For static and dynamic(Template,List) manifests we compare the index of the last downloaded segment to the number of signaled segments
• For dynamic SegmentTimeline manifests the number of available segments changes with MPD updates. We compare the media time of the last signaled segment to the media time of the last downloaded segment.
• For dynamic manifests we can rely on the fact that segments are only allowed to be added to the last period. Once a new period is signaled and the last segment of the current period has been fetched we can conclude that the last segment of the current period has been loaded
• We update the number of signaled segments after each MPD update
• In addition, there is still a buffer finished decision logic implemented in the BufferController. After appending each segment we check if the current buffer is >= the time until the end of the period.

The advantage of the new approach is that we start prebuffering new periods, even if not enough segments are signaled to fill the whole buffer for the current period.

[dsilhavy]

@lkinasiewicz Can you please check if this solves the prebuffering issue you saw with your multiperiod content

[lkinasiewicz]

Yes, this fixes the problem. I removed my changes from this area in #3718.