Implement graceful stopping of app

[joseferben]

Goal

When pressing Ctrl-C when the Sihl app is running, graceful stop should happen.

Todo

0. store ref to current app and start ctx
1. loop forever (in Lwt_main.run)
2. when command finishes, exit loop
3. when SIGINT comes, exit loop
4. call stop app

let stop app ctx = 
  let* () = app.before_stop ctx in
  print_endline "CORE: Stop services"; 
  let* () = Core_container.stop_services ctx app.services in 
  print_endline "CORE: Services stopped"; 
   app.after_stop ctx