refactor objectstore to allow updates

main changes in the api:
- added guid to JobModel.store_job_data. we want our jobs consumers to provide us with a guid, so we know if we need to update or insert.
- removed job_id from JobModel.mark_object_complete parameters. We will use job_guid to link jobs in the objectstore with their counterparts in the jobs db.

open doubts: JobModel.load_job_data is returning the loaded job ids, as required by JobModel.process_objects. Do we still need it for any reason? 

@camd I'll assign this review to you as you seem to be the best person for it :)