Estimate quantity of data that will be pulled / egressed

[TomNicholas]

@betolink was asking if there is a way to keep track in Dask of the accumulative network traffic for pulling data, but I think Cubed should have enough information to know before execution time how many bytes of data will need to be read from storage. It could even maybe know whether this would involve a cross-region egress or not.

Closely related to #334, in fact its arguably a necessary step for that issue.