separate implementations of  incremental index

separate out impl of IncrementalIndex - 
1) OnheapIncrementalIndex - on heap version of incrementalIndex (used by default)
2) OffheapIncrementalIndex - off heap version, uses direct memory for aggregators and storing dimensions.
can we add a comment explaining the code duplication and also the fact it is very non-ideal and will likely be removed in the near future.