Changes to rechunker executors

[tomwhite]

There are some changes in cubed to the executors that derive from rechunker. It would be worth discussing how to share the code for these.

BeamPipelineExecutor and LithopsPipelineExecutor are new and might benefit from living in rechunker. There are changes in pipeline.py that are needed for these executors to work properly.

Cubed no longer uses PipelineExecutors however, since we have a new DagExecutor that is more general. Not sure if this is useful more widely yet.