Refactor Application to use asio signal handling

* Use signal_set as cross platform way of handling SIGINT
* Remove polling on main thread for shutdown.
* Add extra logging for received signal.
* Clean up exit handling on error in setup routines.
* Reuse isStopped() from Stoppable for status (could be isStopping() instead).
* Ctrl-C should now work for standalone mode as well on Windows.