Use the provided list of servers in the room from the `/send_join`
response, since we will not know which users are in the room. This
isn't sufficient to ensure that all remote servers receive the right
device list updates, since the `/send_join` response may be inaccurate
or we may calculate the membership state of new users in the room
incorrectly.
Signed-off-by: Sean Quah <seanq@matrix.org>
