* slab class reassignment still buggy and can crash.  once that's
  stable, server should re-assign pages every 60 seconds or so
  to keep all classes roughly equal.  [Update: fixed now?, but 
  not heavily tested.  Future: make slab classes, with per-class
  cleaners functions.]

* calendar queue for early expirations of items, so they don't push
  out other objects with infinite expirations.