By Robert McMillan Two months ago, BTC-China was growing fast. It was on a blazing trajectory that would see it become the world’s largest Bitcoin exchange in just a couple of months. With Bitcoin, the world’s most popular digital currency, in the midst of an tremendous upswing of its own, BTC was on the verge of hitting it very, very big. But before that, there would be the double-barreled right of passage. First came the extortion attempt, and then the non-stop computer attacks.