Fix multiperiod representation update

Created by: dsilhavy

The goal of this PR is to consider multiperiod manifests when updating the representations in the RepresentationController. The update of the Representations is postponed whenever live delay is larger than availwindow.end - availwindow.start.

However, for multiperiod we might still be playing in the previous period when a new period is introduced and the respective RepresentationController updates the representations. Live delay might be too large so we need to consider the currently playing period as starttime anchor: Math.min(availwindow.start,currenttime)