Make Task type immutable

This change makes it impossible to mutate payload within Handler or
RetryDelayFunc.

doc.go

@@ -9,12 +9,11 @@ The Client is used to register a task to be processed at the specified time.
 
 	client := asynq.NewClient(redis)
 
-	t := asynq.Task{
-	    Type:    "send_email",
-	    Payload: map[string]interface{}{"user_id": 42},
-	}
+	t := asynq.NewTask(
+	    "send_email",
+	    map[string]interface{}{"user_id": 42})
 
-	err := client.Schedule(&t, time.Now().Add(time.Minute))
+	err := client.Schedule(t, time.Now().Add(time.Minute))
 
 The Background is used to run the background task processing with a given
 handler.