Use RowwiseIterator to uniform all data fetch in storage engine.
All objects in storage engine can be read in iterator format.
For example: Segment, Rowset.
This patch implement two generic iterators: UnionRowwiseIterator,
MergeRowwiseIterator. These two class will add iterator as its inputs.
To implement iterators, we define a new class RowBlockV2, all data read
from iterator is in this format. We define a new class other than use
old version's RowBlock is because we want to keep old code work
normally.
