Clean up federation event auth code (#10539)

* drop old-room hack

pretty sure we don't need this any more.

* Remove incorrect comment about modifying `context`

It doesn't look like the supplied context is ever modified.

* Stop `_auth_and_persist_event` modifying its parameters

This is only called in three places. Two of them don't pass `auth_events`, and
the third doesn't use the dict after passing it in, so this should be non-functional.

* Stop `_check_event_auth` modifying its parameters

`_check_event_auth` is only called in three places. `on_send_membership_event`
doesn't pass an `auth_events`, and `prep` and `_auth_and_persist_event` do not
use the map after passing it in.

* Stop `_update_auth_events_and_context_for_auth` modifying its parameters

Return the updated auth event dict, rather than modifying the parameter.

This is only called from `_check_event_auth`.

* Improve documentation on `_auth_and_persist_event`

Rename `auth_events` parameter to better reflect what it contains.

* Improve documentation on `_NewEventInfo`

* Improve documentation on `_check_event_auth`

rename `auth_events` parameter to better describe what it contains

* changelog