hdf5_ep

[svn-r15395] When an attribute was opened twice and data was written with one of the handles,
the file didn't have the data.  It happened because each handle had its own
object structure, and the empty one overwrote the data with fill value.  This is
fixed by making some attribute information like the data be shared in the
attribute structure.

Tested on smirom, kagiso, and linew.