This should fix some tests, as a lot of them are dependent on
deb.debian.org which no longer supports Debian 9 "Stretch".
Instead we use archive.debian.org which will continue to contain
"Stretch" packages for a long time.
Use CDN-backed Debian mirror to make tests run faster hopefully for
everyone. Redirects might be important to know what exactly is going on
when items are being downloaded.
Allow database to be initialized without opening, unify all the
open paths to retry on failure.
In API router make sure open requests are matched with acks in explicit
way.
This also enables re-open attempts in all the aptly commands, so it
should make running aptly CLI much easier now hopefully.
Fix up system tests for oldoldstable ;)
