Address this TODO:

Specifically, UPSERT statements can occasionally hit a fast-path where they perform a blind PutRequest. However, for those that can't (e.g. writing a subset of columns or need to update a secondary index), an initial row scan must be used, just like with UPDATE statements. We should apply a FOR UPDATE row-level locking mode to the initial row scan of an UPSERT statement, in accordance with the enable_implicit_select_for_update session variable.