by Jessica Martini, Christina Bossinakis and Jill Williams LEXINGTON, KY - Led by a $3.3-million son of Gun Runner, the Keeneland September Yearling Sale powered through a record-setting opening session Monday in Lexington. When the dust had settled after the first of two elite Book 1 sessions, 106 yearlings had sold for $69,240,000. The average was $653,208--the highest ever at a single September session--and the median was $537,500. With just 28 horses reported not sold, the buy-back rate was 20.90%. Those figures are ahead of last year's opening session, when...