This is the final piece of the jigsaw for #9595. As with other changes before this one (eg #10771), we need to make sure that we auth the auth events in the right order, and actually check that their predecessors haven't been rejected.
To do this I've reused the existing code we use when persisting outliers elsewhere.
I've removed the code for attempting to fetch missing auth_events - the events should have been present in the send_join response, so the likely reason they are missing is that we couldn't verify them, so requesting them again is unlikely to help. Instead, we simply drop any state which relies on those auth events, as we do at a backwards-extremity. See also matrix-org/complement#216 for a test for this.
