When we turn on explicit sharding, we'll change the CA serial prefix, so
we can know that all issuance from the new prefixes uses explicit
sharding, and all issuance from the old prefixes uses temporal sharding.
This lets us avoid putting a revoked cert in two different CRL shards
(the temporal one and the explicit one).
To achieve this, the crl-updater gets a list of temporally sharded
serial prefixes. When it queries the `certificateStatus` table by date
(`GetRevokedCerts`), it will filter out explicitly sharded certificates:
those that don't have their prefix on the list.
Part of #7094
Jacob Hoffman-Andrews