Define delayed event ratelimit category

[AndrewFerr]

Apply ratelimiting on delayed event management separately from messages.

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)

[hughns]

Please see question inline about the default value.

[fkwp]

@AndrewFerr what is the current status on this topic?

[AndrewFerr]

@AndrewFerr what is the current status on this topic?

I still need to address the review comments, but once that's done, this should be good to go (barring further review). It should also be usable for testing/development until then.

Since this ratelimit targets the simpler management commands of cancel/restart/get, it's less complicated than the ratelimits we want for delayed messages themselves, which does require more thought.

[anoadragon453]

This LGTM!

Complement test failures are a flake. Note that TestDelayedEvents/delayed_state_events_are_kept_on_server_restart has been flakey for a while; could you please fix that in another PR @AndrewFerr :)

[AndrewFerr]

This should be ready to merge now. The only change since the review is b014c07.