hdf5_chunking_query_bmr

[svn-r18445] Purpose: Fix bug 1637

Description:
Modified algorithm for extending a dataset with early allocation so it only
deals with the new chunks.  Formerly, it would loop over all chunks, checking
to see if each existed in cache and on disk, causing major performance issues
with large numbers of chunks.

Tested: jam, linew, amani (h5committest)