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:
f737368a26/synapse/handlers/room_member.py (L518-L540)
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
