Better log messages

[victor-li]

[kc1212]

perhaps this one https://github.com/op/go-logging?

[aaronang]

I think we don't need an external logging library for this. In my opinion, our logging is fine if we adhere to the convention: [methodName] Message.

[kc1212]

The slave logging doesn't always do that, but it's probably fine even if we don't adhere to it.