Kudu rebalancing tool

[William Berkeley]

Hi Kudu devs. I've written up a short doc proposing a design for a manual
rebalancing tool:

https://docs.google.com/document/d/1jEoruO6NF_YykHsqt_GKZR-ExRkYjVE6SZq2McyKYjg/edit?usp=sharing

I'd appreciate any comments or questions.

-Will