server: Refactor `stop()` to avoid no-async-promise-executor lint error
server: Refactor `stop()` to avoid no-async-promise-executor lint error

Also log when Etherpad has stopped.

ether/etherpad-lite,ether/etherpad-lite,storytouch/etherpad-lite,storytouch/etherpad-lite,storytouch/etherpad-lite,ether/etherpad-lite