MSC4242: State DAGs (storage)

[kegsay]

Split out from #19425

Add storage functions needed for federation support for MSC4242:

• get_missing_events_state_dag: will be used to satisfy /get_missing_events requests.
• get_state_dag: will be used to satisfy /send_join requests.

Part of a series of 5x PRs to land the federation part of MSC4242 (storage, fedclient, serving, inbound-joins, inbound-pulls).

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)

[CLAassistant]

All committers have signed the CLA.

[MadLittleMods]

Updated to the following

synapse/synapse/storage/databases/main/event_federation.py

Lines 1313 to 1316 in 3bf561e

	# Sort the seeds lexicographically by event ID: seeds are all 0 hops away from
	# themselves, so the event ID is the only tie-breaker available for them and we need
	# the walk to start from a deterministic order.
	seed_ids = sorted(set(latest_event_ids) - earliest_set)

We should point to MSC4242 that explains this behavior