datastore: Do not enqueue duplicate update jobs

The reconciler schedules replication jobs for repositories that are not up-to-date. It creates a new event each time it notices a repo is outdated. We've seen situations where the job queue gets filled up with a large amount of replication jobs. This is extremely inefficient and makes the table grow to unmanageable sizes.

This change will avoid creation of a new update job when there is already one there that's not being processed yet for the same repo.

Closes: #3940 (closed)