By Martyn Herman RIO DE JANEIRO, Aug 16 (Reuters) - Defending champion Laura Trott extended her lead at the top of the Olympic track cycling women's omnium on Tuesday after posting the second fastest time in the two-lap time trial. After four of the six events the British rider is 12 points ahead of Belgium's Jolien D'Hoore and 16 better off than American Sarah Hammer. Trott is bidding for a second gold medal at the Rio Games, having already won the team pursuit in a world record time. More...