Create a Delta Lake reference table that's had an overwrite transaction

[MrPowers]

A Delta Lake table with an overwrite transaction is a great way to make sure the reader isn't just looking at the underlying Parquet files. Correctly reading this table means the reader is correctly parsing the transaction log.

[edmondop]

This is already possible with the current setup, since the RowCollection has a field for it.
https://github.com/delta-incubator/dat/blob/master/dat/model/row_collections.py#L7