Offheap incremental index

* IncrementalIndex now uses offheap to store aggregates
* GroupBy queries use processing buffer to store intermediate results 
I think for usability, either this should specifically recommend decreasing rowFlushBoundary (or the equivalent for groupBy), or we should try to get rid of rowFlushBoundary by having this condition propagate outwards from add(...) as possibly a new return code, and having that be a trigger to the indexers that they should spill to disk. The latter would be pretty nice since rowFlushBoundary is sort of a magical property right now that people just make up numbers for.