Requires consistent job entries, but does not ensure them

update_builds indirectly calls jobs_sorted_by_dependency. This assumes that the state file's dependency topology a valid DAG. But CI director makes no guarantee that the state file is a valid DAG. In fact, get_candidate_determine_result_jobs is interrupted (e.g. by a socket excetion), an incomplete list of job entries may be added.

We believe this is what happened at 2015-08-17 00:05:15.

One solution would be to always ensure the job list is fully-populated at the beginning of each ci-director invocation.