Andrea Mitchell spent 17 years behind the MSNBC anchor desk. Early last year, she quietly stepped down. She told viewers she wanted more time "in the field." But there was a deeper reason she kept mostly private. Alan Greenspan — the former Federal Reserve Chairman who served 19 years under four presidents — was home battling Parkinson's disease. He needed her. Yesterday, Greenspan died at their Washington, D.C. home. He was 100 years old. They'd been married 29 years. Andrea called him "a giant