Pay-for-Performance Provisions are a "Triumph of Theory Over Experience," Writes Bill Keller in "Carrots for Doctors."    "Pay for performance, or P4P in the jargon, is embraced by right and left. It has long been the favorite egghead prescription for our absurdly overpriced, underperforming health care system. The logic  . . .  If only it worked," writes former New York Times executive editor Bill Keller, ...