Returned empty promises in ghost-server
Returned empty promises in ghost-server

- I have no idea why this is required and I really don't like it
- this change looks superfluous but I had problems with Ghost not
  displaying shutting down messages properly and this is the only
  thing that got it working
- the async-awaits were getting stuck in `await this._cleanup()`,
  and then wouldn't enter into the `finally` block
- oddly, this was only reproducible when a job was running, it would
  shut down normally otherwise
- if I find the solution in the mean time, I'll happily get rid of this

e10/Ghost,bitjson/Ghost,e10/Ghost,TryGhost/Ghost,janvt/Ghost,ErisDS/Ghost,ErisDS/Ghost,TryGhost/Ghost,janvt/Ghost,ErisDS/Ghost,bitjson/Ghost,TryGhost/Ghost