Instead of using an in-memory datastore and dealing with the overhead of
computing datastore keys, creating new blocks, etc, use an in-memory blockstore.
We were ignoring quite a few error cases, and had one case where we weren't
actually updating state where we wanted to. Unfortunately, if the linter doesn't
pass, nobody has any reason to actually check lint failures in CI.
There are three remaining XXXs marked in the code for lint.
