There is a corner in `_check_event_auth` (long known as "the weird corner") where, if we get an event with auth_events which don't match those we were expecting, we attempt to resolve the diffence between our state and the remote's with a state resolution.
This isn't specced, and there's general agreement we shouldn't be doing it.
However, it turns out that the faster-joins code was relying on it, so we need to introduce something similar (but rather simpler) for that.
