Create the database user in a transaction

Create the database user in a transaction

In OC\User\Manager::createUserFromBackend the newly created user is read
using getUserObject($uid, $backend) but that can cause causal read
issues (wrote in DB primary, not yet in secondary).

In OC\User\Database user backend the user cache is unset after the
insert, so it can't be used by getRealUID() (which is called by
getUserObject()).

To avoid that we make sure the user cache is repopulated in a
transaction.

Signed-off-by: Thomas Citharel <tcit@tcit.fr>