Factor out `_get_remote_auth_chain_for_event` from `_update_auth_events_and_context_for_auth` (#10884)

* Reload auth events from db after fetching and persisting

In `_update_auth_events_and_context_for_auth`, when we fetch the remote auth
tree and persist the returned events: load the missing events from the database
rather than using the copies we got from the remote server.

This is mostly in preparation for additional refactors, but does have an
advantage in that if we later get around to checking the rejected status, we'll
be able to make use of it.

* Factor out `_get_remote_auth_chain_for_event` from `_update_auth_events_and_context_for_auth`

* changelog