[jobs-api] replace load with diff_job runtime method
job_load functionality uses should_create_job and
should_update_job functions to determine what action
to take on job load.
This PR replaces most of the load logic with
diff_job runtime method.
This greatly simplifies job change detection and will
be used in future PRs for any comparison related operation.
Depends-On: !101
Bug: T359804
Signed-off-by: raymond-ndibe rndibe@wikimedia.org