Offheap incremental index

* IncrementalIndex now uses offheap to store aggregates
* GroupBy queries use processing buffer to store intermediate results 
I don't think wrapping the aggregator is going to help much. The compute threads still build their own aggregators that access the bytebuffers directly, so there is no easy way to prevent contention there. Let's just keep thing the way they were, and we can work on fixing this separately. Let's focus this PR on just offheap indexing.