WIP: Chunk CSVs using pandas.read_csv

closes #587

- [x] performance diffs
- [ ] newline robustness (e.g., appending to the end of a file that has an existing newline)
- [x] revert dshape inference changes and don't let pandas turn `''` into `nan`
Yes, these are defined as class attributes on `Dialect`. At worst they'll all be `None`.