Clean up some logging in the federation event handler (#10591)

* Include outlier status in `str(event)`

In places where we log event objects, knowing whether or not you're dealing
with an outlier is super useful.

* Remove duplicated logging in get_missing_events

When we process events received from get_missing_events, we log them twice
(once in `_get_missing_events_for_pdu`, and once in `on_receive_pdu`). Reduce
the duplication by removing the logging in `on_receive_pdu`, and ensuring the
call sites do sensible logging.

* log in `on_receive_pdu` when we already have the event

* Log which prev_events we are missing

* changelog