"[A] viable cause of action under ERISA for retaliation requires, to succeed, a strong linkage between a job action or other harmful decision and the participant's request for benefits or effort to protect those benefits. Most of the typical disputes that go on day after day between participants and plan administrators don't rise to this level, no matter how it feels to the particular participant trapped in the dispute. Instead, a viable ERISA claim for retaliation looks much more like the facts of [Perez v. Brain], in which the [DOL recently] recovered several hundred thousand dollars in back pay and other damages for a trio of employees and plan participants who blew the whistle on malfeasance by a plan fiduciary and cooperated with a federal criminal investigation."