Praefect: Collapse duplicate replication jobs

Most of the replication jobs created for updating the repo with the latest changes. There could be a lot of events in the queue for updating the same repository with the latest changes.
Once the first event is completed there is no sense in applying of all the other events as the repository is up to date.
As an improvement we mark all the same jobs (the jobs are equal) as 'completed', so they won't be consumed for the processing.
Now we only need to dequeue the oldest events with only distinct replication jobs.

Closes: #2438 (closed)