Offheap incremental index

* IncrementalIndex now uses offheap to store aggregates
* GroupBy queries use processing buffer to store intermediate results 
It seems like there are a few extra places where in-heap materialization is needed to avoid writing out V8 columns to disk and then converting them. Is it actually faster to do this than to write out the V8 columns and convert them? 'Cause it seems like the heap usage will end up being heavier.

Unless I am reading the old and new code wrong…