NEW YORK (Reuters) - A big mystery seller of futures contracts during the market meltdown last week was not a hedge fund or a high frequency trader as many have suspected, but money manager Waddell & Reed Financial Inc, according to a document obtained by Reuters. Waddell sold on May 6 a large order of e-mini contracts during a 20-minute span in which U.S. equity markets plunged, briefly wiping out nearly $1 trillion in market capital, the internal document from CME Group Inc said. The e-minis are one of the most liquid futures contracts in the world, providing holders exposure...