CSV compression only change shortcut

[jonkeane]

For CSVs (and later when we support them for JSONs), if all that is being changed is compression, we can do a shortcut by compressing or decompressing the files directly instead of routing through a pyarrow dataset.

Doing this ends up being slightly tricky because reading lines and gzipping in python isn't particularly fast, so we should try and find something that doesn't do that (or we could rely on system utilities if they exist...)