HDF5: use native mtime, ctime/btime for created_at, updated_at

We currently use HDF5 attributes for staring created_at and updated_at. HDF5 already provides a mechanism for recording such information automatically via `H5Oget_info`[1]. That times will be tracked must be ensured via the corresponding property set by `H5Pset_obj_track_times`.

[1] http://www.hdfgroup.org/HDF5/doc/RM/RM_H5O.html#Object-GetInfo
[2] http://www.hdfgroup.org/HDF5/doc/RM/RM_H5P.html#Property-SetObjTrackTimes