Fix live segment list

[dsilhavy]

This PR intends to fix various issues regarding the timing model

• When the MPD is updated, segments are removed and new segments are added. When we request segments based on the index, the index of the last requested segment needs to be adjusted as well. The last requested segment will not be located at the end of the segmentlist anymore. The DashHandler has been modified accordingly
• The presentation time calculation for SegmentList and SegmentTemplate did not consider the startNumber. This is fixed in SegmentUtils
• ListSegmentsGetter did not consider the startnumber.
• Add period@start when calculating the availability window for segment timeline manifests without timeShiftBufferDepth
• Always calculate the availabilityRange for segmentTimeline live manifests based on the segments defined in the MPD

This should fix issues observed in //wowzaec2demo.streamlock.net/live/bigbuckbunny/manifest_mvlist.mpd