This should fix#2815 by making sure we actually set the `display_name`
and/or `avatar_url` and create the needed membership event.
To avoid creating a new membership event when starting Dendrite,
`SetAvatarURL` and `SetDisplayName` now return a `Changed` value, which
also makes the regular endpoints idempotent.
* Merge user API databases into one
* Remove DeviceDatabase from config
* Fix tests
* Try that again
* Clean up keyserver device keys when the devices no longer exist in the user API
* Tweak ordering
* Fix UserExists flag, device check
* Allow including empty entries so we can clean them up
* Remove logging
