The Herenton-Cohen race for Congress is a throwdown, but what's it all about, really?...by Jackson Baker...Willie Herenton's congressional campaign kickoff, scarcely more than a week after it occurred, is already a signal event in Memphis history. Herenton, the battle-scarred former mayor, faced a crowd of mainly African-American supporters in the ballroom of the University of Memphis-area Holiday Inn and reminded those present that he had broken down barriers by becoming the city's first elected black mayor in 1991. Evoking his own modest upbringing and the shames of slavery and historical forms of discrimination against blacks…