Simplify Job accessors in the Registry

There is no need to call GetJob for every Job found in the /job namespace, the data is already there.

We can get rid of the additional lookup for the job state if we deserialize the Job from the directory rather than starting with the job/<name>/object key.

The additional call for the payload state can go away if we move the /state/<name> object to something like /job/<name>/payload-state.