Memory leaks fixed.

Skip list (skip_destroy_list())  was not freeing the head.
Dispatcher (dpt_destroy()) was not freeing routine_obj.
Nameserver (ns_destroy()) was not freeing error response.
Zone node (zn_destroy()) was not destroying the skip list of
  additional records.
Zone node (zn_add_ref()) was not deleting the created
  zn_ar_rrsets structure if the value was merged in skip_insert().

Zone database destroying changed to delete zone nodes (because
  not all zone nodes are in the zone data structure).
zds_destroy() modified to have the zone node destructor as param.