Motivation:
To allow clients to reclaim opens/locks lost due-to server reboot nfs
server should store client records on a stable storage. Such client
record store can be as simple as text file and as complicated as a
distributed DB.

Modification:
Introduce BerkeleyDB-based client record store, which keeps track of
clients in key-value db. On start client store will use db from previous
start for a recovery. New clients will be stored in a new DB. After
recovery is complete the old db will be removed and new one will be used
and primary.

Result:
nfs server can be configured to use persistent store to keep track of
nfs clients.

Acked-by: Paul Millar
Target: master