Formalize contract between XArray and the dask.distributed scheduler

[jhamman]

From @mrocklin in pangeo-data/pangeo#5 (comment):

XArray was designed long before the dask.distributed task scheduler. As a result newer ways of doing things, like asynchronous computing, persist, etc. either don't function well, or were hacked on in a less-than-optimal-way. We should improve this relationship so that XArray can take advantage of newer dask.distributed features today and also adhere to contracts so that it benefits from changes in the future.

There is conversation towards the end of dask/dask#1068 about what such a contract might look like. I think that @jcrist is planning to work on this on the Dask side some time in the next week or two.

There is a new "Dask Collection Interface" implemented in dask/dask#2748 (and the dask docs docs).

I'm creating this issue here (in addition to pangeo-data/pangeo#5) to track design considerations on the xarray side and to get input from the @pydata/xarray team.

cc @mrocklin, @shoyer, @jcrist, @rabernat

[jhamman]

closed via #1674