* Use utility methods from gomatrixserverlib, rather than reimplementing them

* Return string rather than pointer to string

* Update gomatrixserverlib

- `s/Server/OutputRoomEvent/` in `consumers` to accurately reflect what is being consumed.
- `s/set/userIDSet/` in `notifier.go` for clarity.
- Removed lying comments.

* Split out notifying /sync requests and calculating sync responses

The logic for notifying /sync requests is about to get really
complicated as we optimise when to wake up requests, so split
out that code into a separate struct to isolate it and make
it easier to unit test.