Motivation:
the NFSv4StateHandler(leaseTime, instanceId, clientStore)  constructor
can be used by NFS servers to control NFSv4StateHandler instance
creation. As extended constructor is package protected, thus control is
unavailable to other modules.

Modification:
make extended NFSv4StateHandler constructor public. Updated javadoc.

Result:
more control on how instance of NFSv4StateHandler created.

Acked-by: Paul Millar
Target: master