GEORGE McGREGOR - Detroit's busiest drummer.   There can't be many people who have seen active military service as a teenager in Korea, played music for some of the most famous doyens and despots in the world, yet enjoyed a long and distinguished career as a musician and record producer in one of the most creative eras of popular music ever. George McGregor has. Over five decades, beginning in the mid 1950s, he played drums on everything from marching bands to Motown with a technical p