Offheap incremental index

* IncrementalIndex now uses offheap to store aggregates
* GroupBy queries use processing buffer to store intermediate results 
Nevermind, I see the buffer is sized based on the aggregators you specified, so it shouldn't actually be able to run out of memory. In that case perhaps this should be a WTF?! exception and include a note to the programmer about the mistake they made.

Although, that makes me think that it would be nice if the buffer size were actually not based on the aggregators * rowFlushBoundary, but were actually just a fixed size, and rowFlushBoundary was the thing that varied. I think that sort of configuration would be more useful to people.