The Madison Avenue impresario once ran a media-buying operation and parlayed his experience there into a role as the ultimate go-between among ad agencies, big-spending clients and the media outlets that crave them. In an explosive arbitration filing that surfaced Tuesday, Kassan alleged UTA had undermined the business since his arrival. For its part, UTA said in a statement that it was about to take legal action against him in Los Angeles. At stake is a little-known