* 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
* Factor more stuff out of `_get_events_and_persist`
It turns out that the event-sorting algorithm in `_get_events_and_persist` is
also useful in other circumstances. Here we move the current
`_auth_and_persist_fetched_events` to `_auth_and_persist_fetched_events_inner`,
and then factor the sorting part out to `_auth_and_persist_fetched_events`.
* `_get_remote_auth_chain_for_event`: remove redundant `outlier` assignment
`get_event_auth` returns events with the outlier flag already set, so this is
redundant (though we need to update a test where `get_event_auth` is mocked).
* `_get_remote_auth_chain_for_event`: move existing-event tests earlier
Move a couple of tests outside the loop. This is a bit inefficient for now, but
a future commit will make it better. It should be functionally identical.
* `_get_remote_auth_chain_for_event`: use `_auth_and_persist_fetched_events`
We can use the same codepath for persisting the events fetched as part of an
auth chain as for those fetched individually by `_get_events_and_persist` for
building the state at a backwards extremity.
* `_get_remote_auth_chain_for_event`: use a dict for efficiency
`_auth_and_persist_fetched_events` sorts the events itself, so we no longer
need to care about maintaining the ordering from `get_event_auth` (and no
longer need to sort by depth in `get_event_auth`).
That means that we can use a map, making it easier to filter out events we
already have, etc.
* changelog
* `_auth_and_persist_fetched_events`: improve docstring
