changes job model to rely more heavily on inheritance and implements synchronization between celery status and sqlalchemy PersistentJob.status.  Also adds basic /jobs/status/<job_id> endpoint

Change-Id: I72f8f0c7752f95edc24ddbe0d78d6f15daaab832
Merge branch 'master' of ssh://gerrit.wikimedia.org:29418/analytics/wikimetrics