"Second, while the OLC opinion concludes that a sitting President may not be prosecuted, it recognizes that a criminal investigation during the President's term is permissible. ...[A] President does not have immunity after he leaves office. And if individuals other than the President committed an obstruction offense, they may be prosecuted at this time. ...[W]e conducted a thorough factual investigation in order to preserve the evidence when memories were fresh and documentary materials were available."