LUCENE-3112 enables adding/updating a contiguous block of documents to
the index, guaranteed (yet, experimental!) to retain adjacent docID
assignment through the full life of the index as long the app doesn't
delete individual docs from the block.
It also makes single-pass grouping possible, when you group by
the "identifier" field shared by the doc block, since we know we will
see a given group only once with all of its docs within one block.
This should be faster than the fully general two-pass collectors we
I'm working on a patch but not quite there yet...