Implement duration change for MPD Validity Expiration events
Created by: robertbryer
Implements the event duration portion of the spec for emsg MPD Validity Expiration events, so the event duration field can be used to set the remaining duration of a live stream. It also allows a stream to be ended at the event time when presentation_time_delta and event_duration are 0.
It does not implement the checking of the message_data field or the presentation_time_delta field to verify that the reloaded manifest has the correct validity