Replaced with types.HeaderedEvent _for now_. In reality we want to move
them all to gmsl.Event and only use HeaderedEvent when we _need_ to
bundle the version/event ID with the event (seriailsation boundaries,
and even then only when we don't have the room version).
Requires https://github.com/matrix-org/gomatrixserverlib/pull/373
* Move currentstateserver API to roomserver
Stub out DB functions for now, nothing uses the roomserver version yet.
* Allow it to startup
* Implement some current-state-server storage interface functions
* Add missing package
