start the market thread and register backtest broker thread
        QAMarket 继承QATrader， QATrader 中有 trade_engine属性 ， trade_engine类型是QA_Engine从 QA_Thread继承

def start_market(self):
        """
        start the market thread and register backtest broker thread
        QAMarket 继承QATrader， QATrader 中有 trade_engine属性 ， trade_engine类型是QA_Engine从 QA_Thread继承
        """
        # 启动 trade_engine 线程
        self.market.start()

        # 注册 backtest_broker ，并且启动和它关联线程QAThread 存放在 kernels 词典中， { 'broker_name': QAThread }
        #self.market.register(self.broker_name, self.broker)
        self.market.connect(self.broker_name)