Change the primary key on the issued_certs table

In retrospect a primary key that has no relation to the certificate is preferred to using the certificate KeyID. The KeyID is also very large for a primary index.
This is a moderately tricky migration, especially for SQLite which has no means of altering the table in this fashion - it involves creating the new table and copying the data.
Order of commands also matters - index names are global in SQLite, so the `idx_expires_at` index needs to be created at the correct stages.

For MySQL migration the necessary steps are run as a single alter statement to minimise the risk of leaving the migration in an incomplete state if anything aborts.

When tested on a table with 250,000 rows (MySQL 5.7) the migration took 3 seconds to complete. As certificates will be requested infrequently the risk of prolonged locking is minimal.

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