In `sa.checkFQDNSetExists`, query both the normal `fqdnSets` and the
`fqdnSets_old` tables. The `fqdnSets` table was recently truncated to
only have 7 days worth of data, but this helper function is used to
bypass other rate limits if there exists a prior certificate for the
exact same set of names, and that functionality cares about at least
90 days worth of data. Therefore we need to query both tables, at least
until `fqdnSets` contains 90 days worth of data again.
Also make a variety of other changes to support this change: creating
the `fqdnSets_old` table in our test environment, documenting various
places where it needs to be cleaned up, and removing some unused code.
Fixes#5671
f21ba0d8a7