DAOS-VOL

Merge pull request #211 in HDF5VOL/daos-vol from ~SONGYULU/daos-vol:master to master

* commit 'f1a284adf4d6347688add70e61a8a1765f8b0a75':
  Adding interval wait to specify how often to do wait after async I/O, -p command-line option.
  Minor change: free a memory allocation.
  Adding MPI_Barrier for each object operation.
  Updated for async I/O support.