Merge pull request #2167 in HDFFV/hdf5 from refactor_optional_vol_callback_02 to develop
* commit 'c2ca33dcfa340af603399036dfddad0ebbd72f6f':
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
Cleanups from PR reviews
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.
