- eliminate qctx->hookdata and client->hookflags.
- use a memory pool to allocate data blobs in the filter-aaaa module,
  and associate them with the client address in a hash table
- instead of detaching the client in query_done(), mark it for deletion
  and then call ns_client_detach() from qctx_destroy(); this ensures
  that it will still exist when the QCTX_DESTROYED hook point is
  reached.