- should re-add pre_load() of index before to lock() some mutex

- should reverse bytes order for address inside node, to be able to 
  test first byte before to test all the following to know if it is empty
  could speed up 0.0001% process :-)


DONE :

- now xstat->h (file header) is saved only on truncate and on file close
  I should save it at regular interval or always to reduce problems 
  when shutdown unexpectedly.



