Refund unused rate limit on downloads

[turt2live]

Fixes #17394 (I think/hope)

A more complete fix likely involves the use of a Linearizer over the download endpoint, limiting the number of concurrent requests to ~6 from any one client. We would then only count against the requester's bucket once we know the size for sure instead of decrementing by max_size. Instead, this PR tries to refund the difference back to the rate limit, allowing the user to request more media at the cost of being rate limited if they have several (assumed) large concurrent requests on the go. The hope is this is uncommon, and that downloads complete quick enough to refund the bucket.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct
  (run the linters)

[turt2live]

I'm not confident enough to say the issue is fully fixed, but it should be better for RC/release purposes.

[turt2live]

Closing in favour of #17439