Move ThirdPartyEntityKind into api.constants so the expectation becomes that the value is significant

Avoid so much copypasta between 3PU and 3PL query by unifying around a ThirdPartyEntityKind enumeration

Wrap the `Requester` constructor with a function which provides sensible
defaults, and use it throughout

s/domian/domain/g

This code is kind of rough (passing the remote servers down a long
chain), but is a step towards improvement.

This reverts commit cf81375b.

It subtly violates a guest joining auth check

There's at least one more to merge in.

Side-effects:
 * Stop reporting None as displayname and avatar_url in some cases
 * Joining a room by alias populates guest-ness in join event
 * Remove unspec'd PUT version of /join/<room_id_or_alias> which has not
   been called on matrix.org according to logs
 * Stop recording access_token_id on /join/room_id - currently we don't
   record it on /join/room_alias; I can try to thread it through at some
   point.

This tracks data about the entity which made the request. This is
instead of passing around a tuple, which requires call-site
modifications every time a new piece of optional context is passed
around.

I tried to introduce a User object. I gave up.

POST /_matrix/client/api/v1/rooms//send/a.b.c gave a 500 error, because we
assumed that rooms always had at least one character.

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.

Make sure the notifier stream token goes forward when it is updated. Sort the pending events by the correct room_stream_id

Extract the id token of the token when authing users, include the token and device_id in the internal meta data for the event along with the transaction id when sending events

fix the copyright holder from matrix.org to OpenMarket Ltd, as matrix.org hasn't been incorporated in time for launch.

Avoid hardcoding names of individual stream token keys in its own implementation; this at least reduces the number of places in source code the individual parts are stored

WIP: Completely change how event streaming and pagination work. This reflects the change in the underlying storage model.

Define __copy__ and __deepcopy__ as identity functions on DomainSpecificString, so that copy.deepcopy() will work on them