MEXICO CITY (Reuters) - Street riots, decapitations of police officers by drug gangs and the worst union conflict in years have raised tension in Mexico's presidential race with the government under fire for its handling of the violence. Thousands of police swarmed a town near Mexico City this week to free fellow officers taken hostage in riots that left a 14-year-old boy dead and led to scores of arrests. The violence, triggered by a dispute with police over unlicensed flower sellers, came two weeks after two steel workers were killed during running battles with police sent in to break a...