Check-in on abandon rules check and abort in-flight frag when stuck on playlist loading

[robwalch]

This PR will...

Check-in on abandon rules check and abort in-flight frag when stuck on playlist loading

Why is this Pull Request needed?

Switches down to a variant that should load at line speed when the higher bitrate in-flight fragment is late. If the playlist of the lower variant still hasn't arrived by the time estimated, and the original in-flight fragment is still active, abort it and switch down further if needed.

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

Fixes redundant switching level / level loading events that occur for VOD when playlist loading is significantly delays. These events cancel the fallback abandon rules check playlist timeout.

Resolves issues:

Resolves #6689 (provided throttled requests do not timeout)

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