Fix and test 'chunks' escape hatch.

[danielballan]

Data is typically chunked using a heuristic. We support a "chunks" key, next to "shape" in the descriptor, as a way to override the heuristic. This has been in the codebase for quite awhile but evidently not ever actually run because it is broken. This fixes it and adds a unit test. The fix was also tested in the wild:

In [1]: from tiled.client import from_profile, show_logs

In [2]: c = from_profile('tes')

In [3]: c['95820398-d829-4088-a023-746a78c01f4f']['primary']['data']['fluor']

Looking forward, we agree that StreamResource will enable chunking. Putting chunks in the descriptor is just an escape hatch for legacy documents.