Database insert / insertMultiple return value behaviour

The behaviour of the methods differs from the documented behaviour. This is at least also for mailbox creation also used and only works (even reliably in this case) by chance.

1. carefully check the behaviour
2. check what behaviour it should have by looking at some uses of the return values
3. implementing & documenting ONE behaviour.
4. this likely incorporates changing the assumptions of code using the methods. (E.g. MailboxGateway::createMailbox)

Some links

• https://mariadb.com/kb/en/last_insert_id/
• https://www.php.net/manual/en/pdo.lastinsertid.php (not to be taken too serious as it just calls the underlying DB last_insert_id)