Add graceful shutdown for requesters and responders

[otothea]

Adds support for callback when calling .close() on a component to allow graceful shutdown.

If a callback is passed then the component will:

• immediately close discovery
• Wait for message queue to drain before calling callback

example use case:

const Responder = require('cote').Responder;

...

process.once('SIGINT', () => {
    randomResponder.close(() => {
        process.exit();
    });
});

[daniandl]

Hate to be that guy, but, what is stopping this from being merged?