Add auto-increment column to orderToAuthz2

[mcpherrinm]

orderToAuthz2 is the only table without an auto-increment column

This requires special handing in partition-manager, which we recently discovered was buggy:

• Partition ordering for multi-column keys is incorrect mariadb-sequential-partition-manager-py#60
• Correct multi-column key partition ordering mariadb-sequential-partition-manager-py#61

We'd like to do away with multi-column key ordering, which requires Boulder have an auto-increment column on orderToAuthz2.

[aarongable]

JC's proposed schema is:

CREATE TABLE IF NOT EXISTS `orderToAuthz2` (
  `id` bigint(20) UNSIGNED NOT NULL AUTO_INCREMENT,
  `orderID` bigint(20) NOT NULL,
  `authzID` bigint(20) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `orderID_idx` (`orderID`),
  KEY `authzID_idx` (`authzID`)
)

Boulder gets unhappy when we replace our current table schema with this:

validateFinalizeRequest: error in GetValidOrderAuthorizations: rpc error: code = Unknown desc = failed to select authz2: Error 1052: Column 'id' in field list is ambiguous

This is because GetValidOrderAuthorizations does a JOIN on the authz2 and orderToAuthz2 tables, but selects columns from that joined result by column name only, not by table-qualified column name:

boulder/sa/model.go

Line 432 in bd1d27b

const authzFields = "id, identifierType, identifierValue, registrationID, status, expires, challenges, attempted, attemptedAt, token, validationError, validationRecord"

boulder/sa/saro.go

Lines 1000 to 1007 in bd1d27b

	fmt.Sprintf(`SELECT %s FROM authz2
	LEFT JOIN orderToAuthz2 ON authz2.ID = orderToAuthz2.authzID
	WHERE authz2.registrationID = :regID AND
	authz2.expires > :expires AND
	authz2.status = :status AND
	orderToAuthz2.orderID = :orderID`,
	authzFields,
	),

So now that both authz2 and orderToAuthz2 have columns with the same name, there's ambiguity when saying SELECT id, ... FROM authz2 LEFT JOIN orderToAuthz2.

We'll need to update GetValidOrderAuthorizations to prepend "authz2." to the names of all the columns its SELECTing.