Delivery is the hottest thing in the restaurant business right now — but as many restaurants are finding, the trouble is keeping it cold. Or crisp, in the case of french fries. Or warm, in the case of pho. Eager to join a booming food- and restaurant-delivery market, dozens of third-party services have sprung up to address those logistical challenges. Food delivery is already a $43 billion business — and will be worth $76 billion by 2022, according to an analysis by Cowen and Co., an investment banking firm.