Add `--no-cleanup` and `--resume` options to `dolt clone`

Currently, if a clone fails, we delete the directory we were cloning into so we don't leave a mess after ourselves. This is suboptimal if debugging a clone failure. Also, for large clones, it would be nice to be able to resume a failed clone.

If the `--no-cleanup` is implmented independently, this would be used for debugging purposes. A `--resume` option would be more complicated but could reuse the chunks we have already downloaded.