Object fetches and WebFinger stop sending requests to a host until
Retry-After or X-RateLimit-Reset (default 5 minutes, max 1 hour).
Ported from Akkoma's Pleroma.HTTP.Backoff.
Co-authored-by: FloatingGhost <hannah@coffee-and-dreams.uk>
Co-authored-by: Oneric <oneric@oneric.stub>
