As it is done in the RPZ module, use 'db' and 'dbversion' for the
database we are going to update to, and 'updb' and 'updbversion' for
the database we are working on.
Doing this should avoid a race between the 'dns__catz_update_cb()' and
'dns_catz_dbupdate_callback()' functions.
(cherry picked from commit d2ecff3c4a)