Eng 6530 txn not span buffers

What we would like is to avoid having duplicate code between DRTupleStream and ExportTupleStream so having extendBufferChain only in TupleStreamBase is a good thing. What we want is to either implement special stuff for DR in additional methods invoked by TupleStreamBase, or alternatively have TupleStreamBase extract what the correct policy is from the derived class and have the entire implementation in TupleStreamBase. I think we want to have the entire thing in TupleStreamBase and just have the derived class signal what kind of buffer chain extending it wants done.