st Cir.2000) (finding no closed-ended continuity where 17 predicate acts of wire and mail fraud occurred over 21-month period); Cofacredit. S.A. v. Windsor Plumbing SupnlY Co., 187 F.3d 229, 242 (2d Cir.1999) (stating that the Second Circuit "has never held a period of less than two years to constitute a `substantial peri