* Inline `_check_event_auth` for outliers
When we are persisting an outlier, most of `_check_event_auth` is redundant:
* `_update_auth_events_and_context_for_auth` does nothing, because the
`input_auth_events` are (now) exactly the event's auth_events,
which means that `missing_auth` is empty.
* we don't care about soft-fail, kicking guest users or `send_on_behalf_of`
for outliers
... so the only thing that matters is the auth itself, so let's just do that.
* `_auth_and_persist_fetched_events_inner`: de-async `prep`
`prep` no longer calls any `async` methods, so let's make it synchronous.
* Simplify `_check_event_auth`
We no longer need to support outliers here, which makes things rather simpler.
* changelog
* lint
