Do not keep rerunning all background jobs

[olevski]

The purpose of some background jobs are to simply fix one-time issues that have cropped up in the regular or authorization database. Therefore we do not need to keep rerunning them all the time - as we currently do.

So we need some way to limit how many times a specific job runs.

@leafty suggested that we just keep track in the database on how many times a job has run and then not run it after some number. But is good to run these multiple times to make sure we avoid synchronization problems or race conditions.

[leafty]

My suggestion would be that for data migration jobs: at the start of the job collect the set of items to process. If that set is empty N=3 times in a row, you have reached consistency and the job doesn't need to run anymore.