"There is more and more a focus in plan draftsmanship on including terms that could limit, either substantively or tactically, the ability of participants or beneficiaries to successfully bring suit, such as the increased use of contractual limitations periods and venue selection clauses, which are both issues that have garnered the attention, to varying degrees, of the Supreme Court.... [P]laintiffs' successes in ERISA litigation over the recent past have really driven plan sponsors and their lawyers to think proactively about what they can do, in writing their plans, to raise the level of difficulty for plaintiffs and their lawyers in ERISA litigation."