Use TextTrackCue "enter" event to improve Interstitial playback timing

[robwalch]

This PR will...

• Use TextTrackCue "enter" event to improve Interstitial playback timing
• Restore DateRange metadata TextTrack cues when re-attaching media

Why is this Pull Request needed?

TextTrackCue "enter" events provide more precision for Interstitial timing than HTMLMediaElement "timeupdate" events.

Are there any points in the code the reviewer needs to double check?

An "enter" event listener is added to TextTrackCues created for interstitial date range asset-list and asset-url attributes. When dispatched, a new hls.js EVENT_CUE_ENTER event is triggered. The interstitial-controller uses EVENT_CUE_ENTER to run its "timeupdate" handler at a more precise start time.

The logic to advance the program is unchanged and still depends on the HTMLMediaElement currentTime.

Resolves issues:

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md