prop224: Refactor rend_data_t so be able to use multiple HS version

Break rend_data_t into something that could looks like this (or maybe without a union...):

struct rend_data_t {
  uint32_t version; /* XXX: Maybe not necessary if our code flow doesn't
                     * require us to learn the version through that data struct. */
  union {
    hs_data_v2_t v2;
    hs_data_v3_t v3;
  } hs;
};

Once we have such construction, we can use v3 with that data structure more cleanly.