A long-awaited appeals court decision threw out a raft of 2014 extortion convictions against two South Boston Teamsters, a reversal that could complicate the prosecution of two top City Hall officials accused of pressuring a music festival to hire union workers.The U.S. Court of Appeals for the First Circuit reversed the finding that Joseph Burhoe and John Perry — members of Teamsters Local 82 in South Boston — extorted businesses holding events at Boston hotels and convention centers by threatening to picket unless they hired union workers.