WIP: Optimize `GitalyClient.feature_enabled?`

What does this MR do?

• Don't run Random.rand() if we're at 100% (Gitaly only)
  100 < 100 is always false so we might be hitting NFS here even at 100% if Random.rand() returns 1 ...
• Reset the percentage when setting the feature to 100%/true (ALL Features...)
  Previously when a feature was at 50%, !feature-set feature true did nothing since the PercentageGate wasn't being reset.
• Enable a feature if set to a percentage.

Are there points in the code the reviewer needs to double check?

The percentage reset in the API-change could probably need an extra eye. Not sure who to ping.

Why was this MR needed?

Annoyance, mainly 🤷

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)