Speed up remote invite rejection database call (#8815)

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:

https://github.com/matrix-org/synapse/blob/f737368a26bb9eea401fcc3a5bdd7e0b59e91f09/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

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/8815.misc b/changelog.d/8815.misc
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+Optimise the lookup for an invite from another homeserver when trying to reject it.
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