MSC4242: State DAGs (CSAPI)

[kegsay]

This implements MSC4242: State DAGs, without support for federation.

A general overview:

• It adds a new room version and new event type.
• It adds a new field calculated_auth_event_ids to internal metadata.
• It stores the state DAG via new state DAG edges / forward extremities tables.
• It adds new auth rules as per the MSC.
• It uses the new prev_state_events field instead of prev_event_ids() when doing state resolution.

Complement tests: matrix-org/complement#841

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)