To constantly re-verify repositories we should find projects
whe the last verification attempt happened before the minimum
re-verification interval. This interval says after how many
days (at least) a repository should be re-rverified.

Scope the queries to retrieve failed repository verifications
by (repository|wiki)_retry_at < Time.now to respect the next
retry attempt, otherwise we retry them before the
(repository|wiki)_retry_at.