Implement GC on the `unprocessed` index

[volmedo]

unprocessed index is an index on the main egress-records table. Many things can go wrong that can cause a batch to be impossible to consolidate (network partitions, data loss, etc.). To avoid items piling up in the index, we should remove those that are older than a given threshold. We should send them to a second index of "never processed items" so that we can investigate why they were not processed in time.