Reverse how columns in JobDependencies are used

They were used in the wrong before. Since everything used the children
and parents relations until now, this never mistake actually manifested.

Unfortunately, this is a non-backwards compatible change. All entries in
the job dependencies table will have to be manually reversed once.
Eliminate per-job query to check for child jobs
Eliminate one extra query per job for tags
Merge pull request #277 from guidow/optimize_jobs_list_again

Optimize jobs list again