This implements the idempotent replay cache need for NFSv4 OPEN state.
each state owner (open owner or lock owner) is required to store the
last sequence number mutating operation, and retransmit it when replayed
sequence number is presented for the operation.

I've implemented the cache as a static buffer of size 112 bytes
(NFSD4_REPLAY_ISIZE) which is large enough to hold the OPEN, the largest
of the sequence mutation operations.  This implements the cache for
OPEN, OPEN_CONFIRM, OPEN_DOWNGRADE, and CLOSE.  LOCK and UNLOCK will be
added when byte-range locking is done (soon!).