iterator.go
func (it *Iterator) Item() *Item {
	tx := it.txn
	tx.addReadKey(it.item.Key())
	return it.item
}
func (it *Iterator) Item() *Item {
	tx := it.txn
	tx.addReadKey(it.item.Key())
	return it.item
}
// Item returns pointer to the current key-value pair.
// This item is only valid until it.Next() gets called.