Bygones? I don't think so

Prosecuting people responsible for torture is not "controversial"  -- it is a legal international treaty obligation to the Geneva conventions for sake.
There have been dozens of torture deaths in U.S. custody ( more listed here) 

Waterboarding is unquestionably torture.
The Tokyo war crimes tribunal tried, convicted and sentenced a Japanese officer to 15 year hard labor for waterboarding GIs in World War II. 

Despite the best efforts of John Yoo and Alberto Gonzales to try to weasel around it, waterboarding is torture. Failure to prosecute for torture is itself a war crime. Obama has no choice -- he must order the Justice Department to investigate and charge people where appropriate.