catz: use two pairs of dns_db_t and dns_dbversion_t in a catalog zone

catz: use two pairs of dns_db_t and dns_dbversion_t in a catalog zone

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 d2ecff3c4a0d961041b860515858d258d40462d7)