revise dns64 module to support multiple module instances

revise dns64 module to support multiple module instances

- this enables the use of more than one dns64 block in the same named.conf.
  instead of having a single hash table and memory pool shared by
  all instances of dns64, set up unique ones for each instance and
  store them in an instance structure which is passed to hook functions
  as action_data.
- also removed some hook actions from dns64 that turned out not to be
  needed.