chore: warn log on conn lost

[dxps]

First of all thank you so much for doing and sharing this MQTT 5 Server. 🙏
At least in my initial searches, this was the only result of a Go(lang) based MQTT 5 Server.
And I'm planning to contribute to it in my following weeks and months.

---

I know it might be just a picky thing and maybe I'm used to see stack traces when something really bad or unknown happened (in which case a stack trace that should bring enough context details).

When I tested initially with a paho.golag client (which works just fine, btw, at least for now), in case of an authentication failure (either user doesn't exist or client provide wrong user credentials), the server (gmqttd) logs the error but also the stack trace. And that's defined on purpose in config/config.go:237) by:
zaplog := zap.New(core, zap.AddStacktrace(zap.ErrorLevel), zap.AddCaller())

Since to me it looks like something really bad happened.

And that's why I'm proposing this small change for now: a connection lost should be logged as warning.
Later, I would include one additional logging config item to specify at which level you'd like to have the stack trace included.

[DrmagicE]

Thanks for the contribution. I agree that warning would be more appropriate in this situation.