aligned: On BlueGene, align to at least 32 bytes
end: Move reference to sparse_store_t from predict to block_store

The facilitates replacing sparse_store_t with simple_store_t.
sparse_store: Fix a comment typo
Replace virtual memory based sparse_store_t with compacting_store_t

BlueGene doesn't have virtual memory, so now we manage everything
ourselves using a thread safe compacting collector.  As a significant
benefit, we will no longer suffer the peak storage cost over different
calls to accumulate().  In addition, since we now allocate the bulk of
the memory up front, other fragmentation is probably less likely.