hdf5_chunking_query_bmr

[svn-r15941] Purpose: Close bug 1305.

Description: Added a new function, H5O_alloc_shrink_chunk, which removes all
null messages from a chunk and shrinks the chunk appropriately.  Modified
H5O_merge_null to call this function when a null message is created with a size
>= 64k.  Tests added for this functionality.

Tested: kagiso (h5committest on 1.8 version)