Use RequestStore to memoize Flipper features so that memoized values are cleared between requests

What does this MR do?

This improves https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18490 by using RequestStore to memoize Flipper features so that memoized values are cleared between requests.

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

Why was this MR needed?

This was discovered by @jacobvosmaer-gitlab while developing locally. That could lead to features toggle not up-to-date until Unicorn processes are restarted.

Fortunately, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18490 hasn't been shipped in a tagged release yet so we're still on time to fix that.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?