R1:fca8b5ab1b36

Improve UX for repository updates

Summary:
Fixes T5926. Fixes T5830. Ref T4767. Users currently sometimes have a hard time understanding repository update frequencies. This is compounded by aggressive backoff and incorrect backoff while importing repositories.

- Don't back off while importing repositories. This prevents us from hanging at 99.99% for inactive repositories while waiting for the next update.
- Back off less aggressively in general, and even more gradually during the first 3 days. This…