WALTHAM — David Lee is finally a Celtic, five years after Danny Ainge first tried to acquire him.On the day that Lee became a free agent five years ago, Ainge called to try to convince him to sign with the Celtics. They couldn’t offer him much f money, however, and he ended up signing a six-year, $79.5 million deal with Golden State.“They said,” Lee remembered, “‘Look, we know you’re going to be getting a lot of big offers, and [...]