`service::users::Data::iter` is possibly bugged

https://gitlab.computer.surgery/matrix/grapevine/-/blob/5a490a4397f0c6a36dab1cb631dadc67a849deab/src/database/key_value/users.rs#L86-100

The comment says:

Returns an iterator over all users on this homeserver.

But it actually includes remote users too.