In the 1870s, a bright young dentist — tall, lean, mustachioed and blonde, with a slight speech impediment and a nagging cough — opened his practice in Deep Ellum.  The lanky Georgia native Henry John Holliday had earned a doctorate of dentistry at 19 and won three awards, including best set of gold teeth, at a Dallas County fair. But Doc, as he was known, had a dark side. Not only was he sick with a terminal illness, tuberculosis, but he also had a gambling habit. Thus, he would never become the doctor he might have been.  Like some other promising healers in this story (most of whom had far…