involved extensive restructuring of the reference counting of
zones and related objects.

Zones now attach to their views.  To avoid a circular dependency that
would keep views from ever shutting down, this is done using the new
functions dns_view_weakattach() / dns_view_weakdetach() which
guarantee that the view will not be freed but still allow it
to be shut down.

The zones themselves now only have a single reference count, with
similar "weak" semantics.  Managed zones must now be shut down
explicitly by calling dns_zone_shutdown().  To shut down all
zones in a zone table, call dns_zt_shutdown().

The zone manager is now reference counted, weakly. To shut down the
zone manager, you must explicitly call dns_zonemgr_shutdown().