The new `StateService` type wraps a `SledState` and a `MemoryState`.
This will allow the sled-related code and the in-memory code to be kept
separate, with the top-level `StateService` making method calls to one
or the other, as appropriate.
This commit removes the existing Service impl for the SledService. This
saves time in refactoring, and the code needs to be rewritten
anyways so there's no loss to deleting it now.