fix golint issues in core/queue (#507)

core/queue/producer.go

@@ -1,15 +1,18 @@
 package queue
 
 type (
+	// A Producer interface represents a producer that produces messages.
 	Producer interface {
 		AddListener(listener ProduceListener)
 		Produce() (string, bool)
 	}
 
+	// A ProduceListener interface represents a produce listener.
 	ProduceListener interface {
 		OnProducerPause()
 		OnProducerResume()
 	}
 
+	// ProducerFactory defines the method to generate a Producer.
 	ProducerFactory func() (Producer, error)
 )