Merge pull request #2229 in HDFFV/hdf5 from vol_changes_to_1_12 to hdf5_1_12
* commit 'fcdcc4beb18cfb6098ec2f0e0d45db38ee59178f':
Remove mismerged code
Cherry pick of 0225e6d5969
Cleanups from PR reviews
Remove unnecessary H5CX call
Refactor H5Dvlen_get_buf_size to use optional dataset operation, with generic fallback for VOL connectors that don't implement operation
Refactor all the 'H5VL_*_optional' callbacks to move the type of operation out of the va_list, so it's at least possible for another connector to know what the operation is and decide whether to implement it or not.
Update h5debug to retrieve file pointer through VOL framework
