Removes device_id and ClientInfo
device_id is never actually written, and the matrix.org DB has no
non-null entries for it. Right now, it's just cluttering up code.
This doesn't remove the columns from the database, because that's
fiddly.
Add logic to map the appservice token to the autogenned appservice user ID.
Add unit tests for all forms of get_user_by_req (user/appservice,
valid/bad/missing tokens)
