Speed up remote invite rejection database call

[anoadragon453]

This is another PR that grew out of #6739.

The existing code for checking whether a user is currently invited to a room when they want to leave the room looks like the following:

synapse/synapse/handlers/room_member.py

Lines 518 to 540 in f737368

	invite = await self.store.get_invite_for_local_user_in_room(
	user_id=target.to_string(), room_id=room_id
	) # type: Optional[RoomsForUser]
	if not invite:
	logger.info(
	"%s sent a leave request to %s, but that is not an active room "
	"on this server, and there is no pending invite",
	target,
	room_id,
	)
	raise SynapseError(404, "Not a known room")
	logger.info(
	"%s rejects invite to %s from %s", target, room_id, invite.sender
	)
	if not self.hs.is_mine_id(invite.sender):
	# send the rejection to the inviter's HS (with fallback to
	# local event)
	return await self.remote_reject_invite(
	invite.event_id, txn_id, requester, content,
	)

It calls get_invite_for_local_user_in_room, which will actually query all rooms the user has been invited to, before iterating over them and matching via the room ID. It will then return a tuple of a lot of information which we pull the event ID out of.

I need to do a similar check for knocking, but this code wasn't very efficient. I then tried to write a different implementation using StateHandler.get_current_state but this actually didn't work as we haven't joined the room yet - we've only been invited to it. That means that only certain tables in Synapse have our desired invite membership state. One of those tables is local_current_membership.

So I wrote a store method that just queries that table instead, and stuck a cache on top. nevermind.

[clokep]

Do we need to invalidate this cache ever?

[anoadragon453]

Yep! And after looking at the places this cache needs to be invalidated... it's a few.

It was a nice thought, but since this is quite a quick query I'm minded to not block a lot of work on a cache here :l