DAOS-VOL

Merge pull request #123 in HDF5VOL/daos-vol from ~NFORTNE2/daos-vol:async_link_obj to async_link_obj

* commit 'b613de5711281394167a72fc97f6906fcfdac86d':
  Add/clarify comments for link iterate by creation order.
  Implement internal asynchrony for link iteration by creation order. Also fix memory leak in link get info, other minor fixes.