The "20160817143417_AddCertStatusNotAfter.sql" db migration adds a "notAfter" column to the certificateStatus database table. This field duplicates the contents of the certificates table "expires" column. This enables performance improvements (see #1864) for both the ocsp-updater and the expiration-mailer utilities.
Since existing rows will have a NULL value in the new field the notafter-backfill utility exists to perform a one-time update of the existing certificateStatus rows to set their notAfter column based on the data that exists in the certificates table.
This follows on https://github.com/letsencrypt/boulder/pull/2177 and requires that the migration be applied & the feature flag set accordingly before use.
Fixes#2237.
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