hdf5_chunking_query_bmr

[svn-r30278] Merge revision 30270 to the 1.8 branch

Fix an issue that could occur when allocating a chunked dataset in parallel,
with an alignment threshold set to be larger than the chunk size but smaller
than the size of the small data aggregator.

Tested: h5committest, jelly, ummon (trunk); h5committest, jelly, ummon (1.8)