instantiate a job instead of fetch

this removes the need for the depended on job.id already being saved
no need for that save anymore
Ensure the FinishedJobRegistry honors an 'infinite' result_ttl of -1
Merge pull request #455 from negz/spotify-freeze

Ensure the FinishedJobRegistry honors an 'infinite' result_ttl of -1
Merge pull request #451 from conslo/depend-on-unsaved-job-ids

Depend on unsaved job ids