[v3] Add test store that instruments traffic to the backend.

[jhamman]

Something we've seen a lot of in Zarr-Python 2 is unexpected traffic between the Group/Array APIs and the store. This ticket tracks the development of an instrumented store that will help us assert that our APIs are keeping traffic to the store to a minimum.

@dcherian has done some recent work on this front. Perhaps we can pull some of that into a test here?