Redid the incremental parsing so that we don’t parse rows we’ve already parsed.
Redid the incremental parsing so that we don’t parse rows we’ve already parsed.

This means we can incrementally load a file, which gives a much faster user experience.

Various timing stats are logged in the console in this commit, comparing the time to parse all rows, vs. the wall time and execution time to parse incrementally with our current incremental strategy.
jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab