Implements part of MSC3944 by dropping cancelled&duplicated m.room_key_request

[matrixbot]

This issue has been migrated from #15842.

---

MSC3944

• m.room_key_request with action: request, if there is a corresponding m.room_key_request message with action: request_cancellation and the same request_id and requesting_device_id fields, sent by the same user after the request was made. If the request message is dropped, the cancellation message is dropped as well. Room key requests can use the same transaction ID if they are requesting the same room key, so a request could be made, then cancelled, and then re-requested. Thus the request(s) sent before the cancellation should be dropped, but the request (if any) sent after the cancellation should be kept.
• m.room_key_request with action: request, if there are other identical requests that are currently in the devices inbox, sent before this request. Room key requests can use the same transaction ID if they are requesting the same room key. This can happen, for example, if a key gets requested, and later re-requested. However, if a device was offline during the initial request and has not received it yet, it is redundant for it to receive both requests. The server only needs to keep the most recent copy (unless it has been cancelled - see above - in which case it does not need to keep any copy).

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.
• Pull request includes a sign off
• Code style is correct
  (run the linters)

Related to #3599.