Use enum replace magic number in Event opeation

[1Ckpwee]

I was confused when I debug here, I didn't understand what the different operations meant until I notice the comments. Maybe define enum for every operation and replace magic number can make it easy to understand. e.g.

// use iota if operation number is progressive
const (
	_ = iota
	xxxOperation
)
// or 
const (
	PingOperation = 1
	ReconnectOperation = 7
	InvalidSessionOperation = 9
	HeartbeatOperation = 11
)
...
switch e.Operation {
	case PingOperation:
		//...
	case ReconnectOperation:
		//...
	default:
		//...
}