Add an index on serials to the revokedCertificates table

[aarongable]

Give the revokedCertificates table a non-unique index on the serial column. This exactly matches the existing index on the certificateStatus table.

We did not include this index initially because we were optimizing this table for the crl-updater's crawl-by-shard-and-expiration behavior, but this index is necessary to look up if a certificate has already been revoked when computing ARI windows and when processing revocation requests.

Part of #8322

IN-11835 tracks the corresponding production schema changes

[github-actions]

@aarongable, this PR appears to contain configuration and/or SQL schema changes. Please ensure that a corresponding deployment ticket has been filed with the new values.

[jsha]

Note: I'd expected to see ADD UNIQUE KEY. But, if I'm remembering correctly, with partitioning we can't use UNIQUE KEYs other than the primary one. So for instance certificateStatus has a KEY on serial but not a UNIQUE one.

For certificateStatus we deal with that by using an UPDATE. That code also protects revokedCertificates from having duplicates. As we switch to revokedCertificates we'll want to make sure that revocations open a transaction, do a SELECT to make sure the row doesn't yet exist, and then INSERT.

[aarongable]

Yep, and I'll do that handling in #8408