Setup downloads on timescaledb

[jonatas]

It's second part of #4642.

Add the downloads and log_downloads table to the timescaledb database.

1. The downloads table represents the gem_downloads but with flat results, every download is a row in the database.
2. The LogDownloads mimics LogTicket and tracks what logs are already processed by timescaledb database.
3. There's a maintenance task that can keep picking pending Logs to be processed.

The actual code makes a duplication of LogTicket and also FastlyLogDownloadProcessor mimics the previous processor. I think we can live with both until we build the trust and guarantee all statistics are working as expected.

TODO

• Add task to migrate data from LogTicket to LogDownloads
• Test this PR with timescaledb disabled and no timescaledb database running
• Make sure when timescaledb is disabled no background jobs or maintenance tasks are scheduled