MEXICO CITY -Mexico is a country in the midst of a deadly turf war between rival cartels and government security forces.One of the symbols of that turf war are checkpoints. Sometimes they are run by municipal police or the military, other times they are run by mysterious masked men. They usually ask you a few questions, take a few notes and you're off to your destination.On Sunday, Claudia Sheinbaum, the leading presidential candidate in Mexico,…