Refactor streams playback start
Created by: bbert
This PR intends to refactor and simplify streams playback start process:
- always use seeking process from PlaybackController to seek controllersevent for initial playback start
- adjust seek time in BufferController according to effective appended ranges (in case of misalignement between manifest and coded fragments)
The changes are illustrated in following sequence diagram: https://sketchboard.me/nCcqJmksDzlb#/