Motivation:
if server uses another non default value for leas time, then value
returned by GETATTR is incorrect.

Modification:
introduce NFSv4StateHandler#getLeaseTime and use it to get the value of
FATTR4_LEASE_TIME attribute.

Result:
a server with non default lease time will return the correct value on
getattr.

Acked-by: Albert Rossi
Target: master