Multi-stage rechunk

[tomwhite]

The rechunker algorithm has support for multi-stage rechunking, which reduces the number of IO operations.

It would be good to enable this in Cubed rechunk. The simplest way to do this would be to expose the min_mem parameter, but it might be interesting to see if there is a way of automatically finding the number of stages that optimizes for something we care about (e.g. min IO ops).