Improve audio video append sync

[robwalch]

This PR will...

Improve AV sync by fixing the use of block/unblock audio append operations in the buffer-controller.

• Audio is considered blocked when the first operation in the audio queue is labeled 'block-audio' (replacing blockedAudioAppend)
• Blocks each audio segment independently, releasing as video advances or moves back (seek)
• Audio start does not need to be known to unblock
• Audio blocking no longer requires an inflight video append
• Audio blocking operations are always removed when executed; they only step forward and append audio when aligned with video (same as before)
• Audio blocking is not stepped through by stepOperationQueue (called when resolving 'aync-block' operations)

Why is this Pull Request needed?

Improves audio/video buffer append cadence and alignment, preventing audio from playing ahead of video in browsers that allow that kind of thing.

Resolves issues:

Resolves #7808

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