By Greg Bishop The 20-year-old middle-distance phenom already has the world’s best time this year. Is a seemingly unbreakable world record next? View the original article to see embedded media. EUGENE, Ore. — At the end of every track race, the faces, bodies and souls of weary, drained participants tell the story of a sport where pain is both endured and expected. Some grimace, eyebrows lowered, noses wrinkled, eyes closed, mouths open. Some slump shoulders; or bend over, hands on knees, gasping for air; or collapse in exhaustion. Then there is Athing Mu, the 20-year-old middle-distance runner…