Demonstrate Columnar batch builder (#602)

* Demonstrate Columnar batch builder

* Correct example, improve chunking

* Simplify constraints

* Form key batches in columnar

Signed-off-by: Moritz Hoffmann <[email protected]>

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Signed-off-by: Moritz Hoffmann <[email protected]>
Co-authored-by: Moritz Hoffmann <[email protected]>

Author: frankmcsherry