Consolidate market_buy with buyy

[zillionare]

currently backtesting/trader-server support two seperate API to place now buy orders, that is market_buy and buy. However, we could be able to merge them into one API, say, keep buy only. and the signature of buy would be:

def buy(security: str, shares: int, price: Optional[float]=None, order_time: Optional[datetime.date]=None, **kwargs)->Dict:
    """Place a buy order.

    The order could be either a market buy entrust or a limit price buy entrust
    Args:
        security (str): 证券代码
        shares: (int): 购买数量
        price (float): 买入价格（限价), 如果price指定为None，将转换为市价买入。
        volume (int): 买入股票数

    Keyword Args:
        limit_price (float): 市价买入时的最高限价

    """