fix structure names in qpcache.c and qpzone.c

fix structure names in qpcache.c and qpzone.c

- change dns_qpdata_t to qpcnode_t (QP cache node), and dns_qpdb_t to
  qpcache_t, as these types are only accessed locally.
- also change qpdata_t in qpzone.c to qpznode_t (QP zone node), for
  consistency.
- make the refcount declarations for qpcnode_t and qpznode_t static,
  using the new ISC_REFCOUNT_STATIC macros.