The download scheduler works as follows:

1. Load a batch of IDs that we need to download from the primary (DB_RETRIEVE_BATCH) into a pending list.
2. Schedule them so that at most MAX_CONCURRENT_DOWNLOADS are running at once.
3. When a slot frees, schedule another download.
4. When we have drained the pending list, load another batch into memory, and schedule the remaining files, excluding ones in progress.
5. Quit when we have scheduled all downloads or exceeded an hour.

Basic support to track the backfilling of repositories

See merge request !1197

Export issues to CSV

Closes #1480

See merge request !1126

This will add a lot of overhead to running specs locally without
Spring. If specs failing locally the person should run `bin/rake
geo:db:test:prepare`.