I was thinking about that too, but since it is not possible to change existing items and the computation of those items must be re-entrant (groupcache just tries to avoid duplicate computations but does not guarantee it), there seems to be no reason for any distributed consensus. In fact, the groupcache design is astonishing simple.
Edit: Scanned the source, looks a like a best-effort distributed lock, rather than any sort of consensus protocol. This works for a cache setting, where e.g. having a split-brain scenario and duplicating the work is no big deal.