Person.setUserId does expensive checks to ensure that user ids are
uniques, but the default id generator already guarantees unicity, so
when default id generator is used we don't need Person.setUserId unicity
checks.

Now when generating user ids, we only consider user id conflict with existing
users, because it's not so expensive and might still happen, for example if
user ids have been migrated from person references when erp5_users PAS plugin
was used.
person.setUserId still performs the expensive checks to prevent duplications
against other transactions using person.setUserId, but not against other
transactions using person.initUserId