code: public Set<K> checkAllPresent(final Iterable<? extends K> keys) {
      Set<K> _keysToLoad = new HashSet<K>();
      for (K k : keys) {
        Entry<K,V> e = lookupEntryNoHitRecord(k);
        if (e == null || !e.hasFreshData(clock)) {
          _keysToLoad.add(k);
        }
      }
      return _keysToLoad;
    }
docs: Checks for entries being present and fresh. Used by {@code loadAll} and {@code prefetchAll}
  
  @param keys keys to check for
  @return keys not present in the cache