Like most Americans, I had never heard of Sheldon Adelson until I read that he and his wife had given more than $10 million to keep Newt Gingrich’s candidacy afloat. The article contained a fleeting mention that he had been the son of a Lithuanian cab driver in Dorchester. That got me wondering: How did he become a Republican? And how much good could he do in Dorchester now - if he cared to?