== TODO ==
+add IV, use it by hashing the long-term readkey with the IV to generate the
+per-version readkey.
improve allocate-and-write or get-writer-buckets API to allow one-call (or
maybe two-call) updates. The challenge is in figuring out which shares are on
-which machines.
+which machines. First cut will have lots of round trips.
(eventually) define behavior when seqnum wraps. At the very least make sure
it can't cause a security problem. "the slot is worn out" is acceptable.