Multiple improvements to performance and responsiveness of restreamer

[ekimekim]

when requesting the same very large playlist over and over.

Main highlights:

• generating the hls playlist is now a streaming result
• if multiple calls for the same time range come in concurrently, we only run it once
• we yield to the event loop in more places to avoid locking up other requests

[ekimekim]

Doing clean up of open PRs. I did some self-review, and I figure that "me, 5 months later" is sufficiently removed to count as a second set of eyes.