txn: add visibility check to cursor
in nessDB, there is 4 isolations:
a) TXN_ISO_SERIALIZABLE, we just to get the last one
b) TXN_ISO_READ_UNCOMMITTED, this is also last one
c) TXN_ISO_READ_COMMITTED, get the latest committed value
d) TXN_ISO_REPEATABLE, get the lastest committed value who is in the clone