Robert Parsons, Rick Foucheux and Sarah Zimmerman in “The Rivalry.” Photo by T.Charles-Erickson. It was a classic case of the lanky, brainy, agitating underdog versus the stouter, more experienced, more appeasement-minded line-tower. But must we reopen O’Brien v. Leno while the wounds are still fresh? Then let us confab instead, my fellow Americans, about The Rivalry, Ford’s Theatre’s often-clunky, still-stirring remount of radio great Norman Corwin’s 1958 dramatization of the Lincoln-Douglas debates of a…