hdf5_ep

[svn-r18868] Purpose: Fix bug in dataset shrinking algorithm

Description:
Previously, it was possible for a chunk to be flushed due to chunk operations in
the callback from H5B_iterate in H5D_chunk_prune_by_extent.  Because flushing
the chunk can force it to be reallocated if it is filtered, this can change the
contents of the chunk b-tree in the middle of H5B_iterate.  Because H5B_iterate
uses a locally cached copy of the b-tree, this causes subsequent operations
to be passed incorrect data.  Rewrote H5D_chunk_prune_by_extent to avoid
H5B_iterate entirely.

Also fixed a bug in the dataset expand algorithm that could cause extra chunks
to be created.

Tested: jam, linew, smirom (h5committest)