API Design for Xarray Backends

It has come time to formalize the API for Xarray backends. We now have the following backends implemented in xarray:

* currently does not inherit from backends.AbstractDatastore

And there are conversations about adding additional backends, for example:

• TileDB: https://github.com/pangeo-data/storage-benchmarks/issues/6
• PseudoNetCDF: #1905

However, as anyone who has worked on implementing or optimizing any of our current backends can attest, the existing DataStore API is not particularly user/developer friendly. @shoyer asked me to open an issue to discuss what a more user friendly backend API would look like so that is what this issue will be. I have left out a thorough description of the current API because, well, I don't think it can done in a succinct manner (thats the problem).

Note that @shoyer started down a API refactor some time ago in #1087 but that effort has stalled, presumably because we don't have a well defined set of development goals here.

cc @pydata/xarray

Start by reviewing the current DataStore API and the stalled API refactor in issue #1087, then compare the listed read and write backends. Done means the project has agreed development goals and a formalized, more user- and developer-friendly backend API.