Bugfix/ll hls endofstream

[cjpillsbury]

This PR will...

Fix a couple of end of stream issues with ll-hls content to ensure ended will still occur.

I've also created a simple TimeRangesMock for the testing side.

Since this is an ENDLIST/end of stream issue, I can't provide a test stream, but I can assist with starting/ending ll-hls streams for reviewers if needed (I'm in Chicago/US Central Time for a rough sense of availability)

Why is this Pull Request needed?

The conditions for detecting end of stream for are insufficient ll-hls content, which results in a stall instead of ended. Part of this is related to the fact that fragCurrent for playlists with parts may in fact be "ahead" of the current endSN. Part of this is because FragmentTracker/its usage to determine endOfStream does not account for parts.

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

I'm currently relying on the levelDetails.partList to detect end of stream conditions.

1. I'm only checking the last part, but I believe it's possible that there could be a gap in the current implementation where hypothetically e.g. lastPart - 1 should also be checked and is missing from the buffer but lastPart is not.
2. Aside from some of the basic conditional checks, this feels like it belongs in FragmentTracker. Unfortunately, there isn't equivalent modeling for activeParts when they're buffered, evicted, etc, so this would be a much larger effort. I've confirmed that we could check the last activePart against the relevant timeRanges in getState(), but this feels too operationally intensive for that method, something like

// ...
  else if (this.activeParts && this.activeParts.length) {
      const { activeParts } = this;
      const lastActivePart = activeParts[activeParts.length - 1];
      const partFragKey = getFragmentKey(lastActivePart.fragment); // or filter for all activeParts w/same key as fragment
      if (partFragKey === fragKey) {
        const timeRange = getTimeRangeForType(fragment.type);
        if (this.isBuffered(lastActivePart.start, lastActivePart.start + lastActivePart.duration) {
          return APPROPRIATE_STATE/STATES;
        }
      }
  }

Resolves issues:

Fixes #4463

Checklist

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

[robwalch]

👍