This should fix#3004 by making sure we also update our in-memory ACLs
after joining a new room.
Also makes use of more caching in `GetStateEvent`
Bonus: Adds some tests, as I was about to use `GetBulkStateContent`, but
turns out that `GetStateEvent` is basically doing the same, just that it
only gets the `eventTypeNID`/`eventStateKeyNID` once and not for every
call.
