By Angela Moon and Doris Frankel NEW YORK/CHICAGO (Reuters) - Within 36 hours of Warren Buffett's announcement of a deal to buy H.J. Heinz, U.S. authorities froze an account linked to possible insider trading. The speed of the crackdown on a lucrative options bet, combined with successful prosecutions of insider trading rings, suggested that regulators were quickly jumping on any suspicious activity. Yet some veterans of the options business are unconvinced. They worry that very profitable options trading ahead of big corporate news is undermining investor confidence in the fairness of ...