MSC4140: finalised delayed events, and more

[AndrewFerr]

MSC4140: finalised delayed events, and more

• Store sent/cancelled/failed delayed events, i.e. finalised delayed events, and support looking them up to inspect whether a delayed event was sent or not. Set limits on how many finalised events to store.
• Support looking up delayed events by ID
• Return 200 when retrying the same action (send/cancel) on an already-finalised delayed event, or 409 for a conflicting action
• Limit how many delayed events a user may have scheduled at a time

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)