Restore dependent job pointers on slurmctld startup to avoid race

dep_ptr->job_ptr is set in update_job_dependency() and in
test_job_dependency(), and is required to test for circular dependencies
in _scan_depend(). On slurmctld startup, we weren't restoring that
because it would just be set in test_job_dependency(), which is
called whenever Slurm tries to schedule a job. However, if a job
dependency was updated before that then dep_ptr->job_ptr wasn't
initialized for all jobs, and therefore it was possible to create a
circular dependency.

Bug 8492