hdf5_der

Merge pull request #165 in HDFFV/hdf5 from ~KOZIOL/hdf5:develop_merge_revise_chunks_09 to develop

* commit '3b0c2b24da5689990c4bc0fcd3afecdf063086c8':
  Bring over support for retrying metadata cache entry loads, along with all the supporting metadata cache callback changes, etc.