When leaving the last room shared with a remote user, the remote user is missing from device_lists.left in the /sync response, even when lazy_load_members is off

[squahtx]

1. Have a room with one local user, @alice:local and one remote user, @bob:remote.
2. Have @alice:local leave the room.
3. @alice:local does not observe @bob:remote in the device_lists.left section of /sync.
4. [If @bob:remote then updates their device list and @alice:local does a /keys/query request, the correct response is returned. This part is working.]
5. [If @alice:local rejoins the room, I believe device_lists.changed will contain @bob:remote, so nothing should break here.]

Not tested with lazy_load_members yet. I presume it's similarly buggy and even trickier to fix.

[squahtx]

This might only happen when the local homeserver is no longer in the room. Synapse uses the set of members currently in the room at /sync time to drive device_lists.left:

synapse/synapse/handlers/sync.py

Lines 1514 to 1517 in 5e7847d

	# Now find users that we no longer track
	for room_id in newly_left_rooms:
	left_users = await self.store.get_users_in_room(room_id)
	newly_left_users.update(left_users)

When Synapse is no longer in the room, it no longer has the current state and so doesn't know which remote users to include in device_lists.left at /sync time.

Also it's rather dubious to be using the current members of the room rather than the members at the time the local user left.

[squahtx]

There is a complement test covering this bug in: matrix-org/complement#459