Warning Signs About Another Giant Bitcoin Exchange SAN FRANCISCO — As the price of Bitcoin has soared, the virtual currency has edged toward the mainstream. Square, the fast-growing payments company run by the Twitter co-founder Jack Dorsey, has begun selling Bitcoins to ordinary consumers, and the Chicago Mercantile Exchange will soon allow banks to trade on the value of Bitcoin. But if you want to see where the price of Bitcoin is actually determined in round-the-clock bidding, you have to…