prop224: Optimize hs_circ_service_get_intro_circ() digest calculation

Our prop224 function for getting an intro circ given its intro object re-calculates the relay identity fpr all the time:

  if (ip->base.is_only_legacy) {
    uint8_t digest[DIGEST_LEN];
    if (BUG(crypto_pk_get_digest(ip->legacy_key, (char *) digest) < 0)) {
      goto end;
    }
    circ = hs_circuitmap_get_intro_circ_v2_service_side(digest);

We could shove that in the hs_service_intro_point_t object as well to cut some digest calculations.