This commit fixes parameters broken in the latest version of `adldap2`.
Specifically:
* `adldap` auth parameters have changed in the latest version.
* "usernames" has become `identities` and `discover` has changed to
`discover_users_by`, `auth` has changed to `bind_users_by`
* Add the missing objectguid field to the users table for adldap2.
* This is added as a nullable (optional) field at the moment to support
tracking LDAP users as adldap2 wants to.