Feature compute separation

@s-u, opening a PR on this to discuss
@s-u, I think of the compute process as being subordinate to the control process, so it seems weird to me that the client has to coordinate this.  Right now I'm adding another case where when the client creates a notebook, it then has to ask the compute process to load it (because otherwise it has no notebook loaded at all).  Let's talk this through next time we meet.