CHANGE: Refresh list of users before trying to add a new one

When having multiple server instances, the list of users can be out of
sync. So it's necessary to refresh the list of users before trying to
add a new one.
CHANGE: If User is already in, get it's UID

In case of multiple service instances running against the same DB, it
happens that there are concurrency issues: the user fails to be added
again because it's already added by another thread. This patch checks
that, and treats the case properly.
Merge pull request #1430 from sposs/FCFixUserManagement

FIX: user management