Process Event: 'exit' listener must only perform synchronous operations

Chapter 10 - Service Registry
[unregisterService](https://github.com/PacktPublishing/Node.js_Design_Patterns_Second_Edition_Code/blob/master/Chapter10/05_service_registry/app.js#L19) listener is asynchronous
[Process Event: 'exit'](https://nodejs.org/api/process.html#process_event_exit) listener must be synchronous