By Martyn Herman LONDON, April 19 (Reuters) - Mark Cavendish is edging closer to selection for Britain's Olympic track cycling team after Jon Dibben, a rival for selection to contest the omnium, underwent elbow surgery on Tuesday. The 30-year-old, the most prolific sprinter on the road with 26 Tour de France stage wins, has been sweating on the chance to finally win a first Olympic medal ever since this year's world championships in London. Cavendish finished sixth in the multi-discipline omnium event at the Olympic velodrome, missing the top-three target Britain's technical director Shane Sutton had set him to secure his place on the plane to Brazil. More...