code: public boolean isNodeAvailable( @Nonnull final K key ) {
          final ManagedItem<Boolean> item = _map.get( key );
          if ( item == null ) {
              return updateIsNodeAvailable( key );
          } else if ( isExpired( item ) ) {
              _map.remove( key );
              return updateIsNodeAvailable( key );
          } else {
              return item._value;
          }
      }
docs: Determines, if the node is available. If it's not cached, it's loaded
  from the cache loader.
  
  @param key
  the key to check
  @return <code>true</code> if the node is marked as available.