Logout for a Guest User fails

This issue has been migrated from [#2173](https://github.com/matrix-org/synapse/issues/2173).

---

Attempting to use the logout endpoint if registered as a guest fails with:

```
{"errcode":"M_UNKNOWN","error":"No row found"}
```

This seems to occur because the `access_token` for a guest user is in fact stored in the `user_ips` and never in the `access_token` table.

I cannot find a specification that suggests Guests should *not* be able to invalidate their access tokens (http://matrix.org/docs/spec/client_server/r0.2.0.html#post-matrix-client-r0-logout).