TupleBuffer2 memory consumption

[jnidzwetzki]

The TupleBuffer2 is used to store large amounts of tuples which might be swapped out to disk, if a certain memory limit is hit. Unfortunately, when the tuple are read by the TupleBuffer2Iterator::TupleBuffer2Iterator, all tuples are loaded into memory. This should be changed to reduce the amount of used memory. Only one (or a small amount) of tuples should be loaded into memory during the iteration.