Queue up federation PDUs while a room join is in progress

Implement no op for room stream in sync

This just takes the existing `room_queues` logic and moves it out to
`on_receive_pdu` instead of `_process_received_pdu`, which ensures that we
don't start trying to fetch prev_events and whathaveyou until the join has
completed.

Add fallback to last_active_ts if it beats the last sync time on a presence timeout.

Format presence events on the edges instead of reformatting them multiple times

Handle PartialDownloadError in CAS login

Fix assertion to stop transaction queue getting wedged

... and update some docstrings to correctly reflect the types being used.

get_new_device_msgs_for_remote can return a long under some circumstances,
which was being stored in last_device_list_stream_id_by_dest, and was then
upsetting things on the next loop.

Add some metrics on notifier

Phone number registration / login support v2

to correctly reset the context when we fire off a deferred we aren't going to
wait for.

Fix routing loop when fetching remote media

Reduce number of spurious sync result generations.

Speed up public room list

- don't blindly proxy all HTTPRequestExceptions
- log unexpected exceptions at error
- avoid `isinstance`
- improve docs on `from_http_response_exception`

Give CodeMessageException back its `msg` attribute, and use that to hold the
HTTP status message for HttpResponseException.

Fix current_state_events table to not lie

If we try and persist two state events that have the same ancestor we
calculate the wrong current state when persisting those events.

Implement delete_devices API