Sliding Sync: Lazy-loading room members on incremental sync (remember memberships)

[MadLittleMods]

Lazy-loading room members on incremental sync (and remember which memberships we've sent down the connection before)

Fix #17804

Follow-up/alternative to #17806

Depends on #17785

Pull Request Checklist

• Pull request is based on the develop branch
  • Wait for Correctly changes to required state config in sliding sync #17785 to merge
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[MadLittleMods]

It's possible that we could remember the specific state_keys that we have sent down before but this currently just acts the same as if a whole type was removed (same as simple_remove_type)

This is just a performance optimization though and the result would look like following:

Suggested change

	{},
	{
	EventTypes.Member: {
	"@user3:test",
	}
	},

Perhaps it's good that we "garbage collect" and forget what we've sent before for a given type when the client stops caring about a certain type 🤷.

[erikjohnston]

I think this makes sense! I am slightly worried about the increase in the size of the tables if we keep adding more and more members in there, but I'll have a little think about how we might make that more efficient.

[erikjohnston]

Suggested change

	expanded_required_state_map[EventTypes.Member] = (
	expanded_required_state_map.get(
	EventTypes.Member, set()
	)
	| {user.to_string()}
	)
	expanded_required_state_map.setdefault(
	EventTypes.Member, set()
	).add(user.to_string())

I think is a lot clearer

[MadLittleMods]

It is but expanded_required_state_map is a shallow copy (expanded_required_state_map = dict(room_sync_config.required_state_map)) and I don't want to mutate the state_key sets in the other map.

There is a comment above (Make a copy so we can modify it. [...]) and I also had something here before when I also made a copy for the state_keys but changed it to just making a new set. I could add a comment here on why we're doing it this way (added ✅).

(it's also an AbstractSet so have to use the operators on it instead of methods)