ocsp-updater: exploit isExpired index for revoked query. (#4032)
ocsp-updater: exploit isExpired index for revoked query. (#4032)

Before modifying the `findRevokedCertificatesToUpdate` query to include `NOT
isExpired` the query shows no `possible_keys` in `EXPLAIN` output.

```
MariaDB [boulder_sa_integration]> explain SELECT serial, status, ocspLastUpdated, revokedDate, revokedReason, lastExpirationNagSent, ocspResponse, notAfter, isExpired FROM certificateStatus WHERE status = 'revoked' AND ocspLastUpdated <= revokedDate LIMIT 1000;
+------+-------------+-------------------+------+---------------+------+---------+------+------+-------------+
| id   | select_type | table             | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+------+-------------+-------------------+------+---------------+------+---------+------+------+-------------+
|    1 | SIMPLE      | certificateStatus | ALL  | NULL          | NULL | NULL    | NULL |  208 | Using where |
+------+-------------+-------------------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.01 sec)
```

Afterwards we see `isExpired_ocspLastUpdated_idx` is considered:

```
MariaDB [boulder_sa_integration]> explain SELECT serial, status, ocspLastUpdated, revokedDate, revokedReason, lastExpirationNagSent, ocspResponse, notAfter, isExpired FROM certificateStatus WHERE NOT isExpired AND status = 'revoked' AND ocspLastUpdated <= revokedDate LIMIT 1000;
+------+-------------+-------------------+------+-------------------------------+------+---------+------+------+-------------+
| id   | select_type | table             | type | possible_keys                 | key  | key_len | ref  | rows | Extra       |
+------+-------------+-------------------+------+-------------------------------+------+---------+------+------+-------------+
|    1 | SIMPLE      | certificateStatus | ALL  | isExpired_ocspLastUpdated_idx | NULL | NULL    | NULL |  208 | Using where |
+------+-------------+-------------------+------+-------------------------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

MariaDB [boulder_sa_integration]>
```
letsencrypt/boulder,bmw/boulder,letsencrypt/boulder,jcjones/boulder,jsha/boulder,jfrazelle/boulder,bmw/boulder,jsha/boulder,jfrazelle/boulder,jfrazelle/boulder,jsha/boulder,letsencrypt/boulder,jcjones/boulder,bmw/boulder,jfrazelle/boulder,jcjones/boulder,jcjones/boulder