Merge pull request #2576 in HDFFV/hdf5 from ~NFORTNE2/hdf5_naf:async_req_fix to async
* commit '3584ba553bdac56847fb279f65923bce9d0a5f83':
Fix bugs in async file routines. Rename H5VL_create_object_generic to H5VL_create_object.
Fix bug in error handling in H5Fclose.
Free the requests is H5ESclose. Remove comments implying that request wait, notify, and cancel callbacks should free the request.
Modify async implementation to wrap async requests in a H5VL_object_t struct so the VOL layer can find the connector when accessing the request at a later point.
