Reload job graph

[reynoldsbd]

It is possible for a job to have side effects which necessitate reloading the entire job graph. Add a new Outcome::Reload variant (see #4) representing this scenario.

When a job returns the reload outcome, stop executing jobs and load a new Schedule based on the originally submitted job. Then restart execution.

As an optimization, it would be interesting if we could somehow preserve the state of the old (pre-reload) schedule and map it onto the new reloaded schedule. Intent of this would be to prevent unnecessary/duplicate execution of jobs after a reload.