WIP: Chunk CSVs using pandas.read_csv

closes #587

- [ ] 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`
Ideally people who are not like you, me, or @eriknw should also be able to read our code.