Use the MIN aggregation function for cert-checker's start-of-window (#5406)

Switch from using `ORDER BY` and `LIMIT` to obtain a minimum ID from the
certificates table, to using the `MIN()` aggregation function.

Relational databases are most optimized for set aggregation functions,
and anywhere that aggregations can be used for `SELECT` queries tends to
bring performance improvements. Experimentally this is an
order-of-magnitude improvement in query time. Theoretically the query
optimizer should have constructed the same underlying query from each,
but it didn't.

Partially reverts #5400
Fixes of #5393

cmd/cert-checker/main.go

@@ -128,7 +128,7 @@ func (c *certChecker) getCerts(unexpiredOnly bool) error {
 	}
 
 	initialID, err := c.dbMap.SelectInt(
-		"SELECT id FROM certificates WHERE issued >= :issued AND expires >= :now ORDER BY id ASC LIMIT 1",
+		"SELECT MIN(id) FROM certificates WHERE issued >= :issued AND expires >= :now",
 		args,
 	)
 	if err != nil {