Flush MSE rendering pipeline when crossing video buffer holes with audio

[robwalch]

This PR will...

Flush MSE rendering pipeline when crossing video buffer holes with audio.

Why is this Pull Request needed?

Chrome will play past a hole in the video buffer when there is audio, but video will not render and playback will stall one second past the hole. https://issues.chromium.org/issues/40280613#comment10

These changes detect playback traversal of such a gap and issue a seek to flush the pipeline render. This prevents the stall and results in video frames being rendered that would not have otherwise (#5631).

The improvement in stall detection and immediate triggering of "bufferNudgeOnStall" when the media element is "waiting" addresses stalls on audio buffer gaps (#6169).

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

Are other gap-controller labelled issues addressed?
https://github.com/video-dev/hls.js/issues?q=is%3Aopen+is%3Aissue+label%3Agap-controller

The gap-controller now resembles other hls.js controllers in that it uses hls.js and media element events as well as a task interval to perform work. There are still a couple of places where it is dependent on the stream-controller that could be improved. For the most part this removes shared responsibility between the stream-controller and gap-controller.
Ex: all responsibility for emitting MEDIA_ENDED is contained in the gap-controller now.

Resolves issues:

• Buffer hole on level switch (Chrome) #5631
• hls.js stalls shortly on audio buffer gap #6169

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