feat: Support Parallel Segment Fetching

[tyrelltle]

closes #4658.

This solution is inspired by abandoned PR #2809, which implements segment prefetching ahead of current play head.

[github-actions]

Incremental code coverage: 98.17%

[avelad]

Thanks for your patience, I'll try to review it throughout this week.

[joeyparrish]

I skimmed this PR, and it looks interesting and well-constructed. You've designed something that appears to address most of my concerns, it defaults to 0 (good for low-memory devices), and the prefetcher cleans up after itself so it doesn't hold onto an unbounded number of segments. This could also be useful in a future implementation of #880 (preload API).

I haven't read it in detail, and I have some deadlines to get AV sync fixes done this week before we go on holidays at Google. But I will come back to this in January for a detailed review, and I expect this is something that will ultimately ship in Shaka Player.

Thanks for your contribution!

[tyrelltle]

Thanks @joeyparrish !
Assuming this change goes into main branch in January, when a new version that has the change will be released ?
Just trying to understand the overall release process.

Thanks again for helping on the code reviews !

[joeyparrish]

We can make a feature release at any time. We don't have a fixed release cycle.

[avelad]

@tyrelltle can you rebase and resolve the conflicts? Thanks!

[tyrelltle]

@avelad yes I just rebased now

[tyrelltle]

@theodab thanks for the comments !
Agree all of them and will make sure address them early next week

[tyrelltle]

@theodab thanks for the comments ! Agree all of them and will make sure address them early next week

addressed some of the comments, will finish other ones by tomorrow

[theodab]

Thanks for the work, by the way.

[tyrelltle]

Thanks for the work, by the way.

Thanks for the comments I updated all of them. Let me know if any other suggestions :)