make most node-core-backing types private

Now the only public types exposed in that module are
CandidateBackingSubsystem and ToJob. While ideally we could reduce
the public interface to only the former type, that doesn't work
because ToJob appears in the public interface of CandidateBackingSubsystem.

This also involves changing the definition of CandidateBackingSubsystem;
it is no longer a typedef, but a struct wrapping the job manager.