"Retirement plans are regular and sophisticated users of swap contracts in their investment portfolios. These plans are also regularly subject to the fiduciary responsibility and prohibited transaction rules of ERISA or the Internal Revenue Code. A concern had arisen about the potential ERISA consequences of the participation by retirement plans in mandatory clearing, including whether a Clearing Member could be deemed an ERISA 'fiduciary,' subject to ERISA statutory obligations to act exclusively for a retirement plan in a manner fundamentally inconsistent with its obligations as a clearing member."