Set limit based off chunksize

[jsignell]

• [ ] Closes #xxxx
• [ ] Tests added
• [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [ ] New functions/methods are listed in api.rst

If you know what the chunksize is on-disk, then when you are using "auto" you can use that as limit for chunksize so that you are less likely to split up chunks.

A more complete, but probably more complex option would be to put something like this into dask itself so that auto_chunks could have a "don't split" option.