The column reader passed to `flux.Table.Do` is automatically released.
The function passed to the column reader should never release it
manually. This causes a double release which causes the table to be
erroneously freed when it might be referenced by another transformation.
In particular, this affected the following:
tables
|> yield()
|> to()
This is because this would produce a buffered table with two references
and pass it to both `yield()` and `to()` because `yield()` is a
pseudo-node that doesn't really exist. The real graph looks more like:
tables |> yield()
tables |> to()
The `yield()` would double release which would release the `to()`
transformation's copy of the column readers. The `to()` method would
then be invoked with an invalid column reader.
Christopher M. Wolff
Stuart Carnie
Jonathan A. Sternberg
Lorenzo Affetti