func (wb *WriteBatch) Set(k, v []byte, meta byte) error {
	e := &Entry{Key: k, Value: v, UserMeta: meta}
	return wb.SetEntry(e)
}
func (wb *WriteBatch) Set(k, v []byte, meta byte) error {
	e := &Entry{Key: k, Value: v, UserMeta: meta}
	return wb.SetEntry(e)
}
// Set is equivalent of Txn.SetWithMeta.