一，Consumer实例初始化
DefaultMQPushConsumer#start()
DefaultMQPushConsumerImpl#start()
二，拉取消息
RocketMQ中推送与拉取消息底层皆为拉取
PullMessageService#run()
DefaultMQPushConsumerImpl#pullMessage()
三，消费消息组装
PullMessageProcessor#processRequest()
DefaultMessageStore#getMessage()
PullMessageProcessor#processRequest()
