Offheap incremental index

* IncrementalIndex now uses offheap to store aggregates
* GroupBy queries use processing buffer to store intermediate results 
Grammar? "Buffer full, cannot add more rows! Current rowSize[%,d]."

Also this is kind of an unceremonious exit, it'll probably make people's realtime indexings crash or drop data if memory overflows. It would be better to pause, I think. Is there some way to do that? Maybe a new kind of return value for "add" that the plumbers can use to trigger a spill to disk?