- Make JCR a class and implement inc_use_count() and

  dec_use_count() methods that ensure that the jcr is
  locked when inc/dec the use count.
- Remove the global jcr lock when traversing the jcr
  chain.
- Use dlist to implement the jcr chain rather than hand
  crafted next and prev links.
- Lock the jcr chain inside each function that modifies
  the chain.