state/multiwatcher: move top level MachineJob type

This PR cleans up the types that were temporarily moved to the top of the project during the `apiserver/params` / `state` transition.

`multiwatcher.MachineJob` is a terrible place to place this type, but it is currently required because it is referenced from the `multiwatcher` types like `MachineInfo`.

The next branch will clean up more of this nonsense.

(Review request: http://reviews.vapour.ws/r/503/)