Change the generic type of List in InternalScanner.next

Change InternalScanner.next method to accept List<? super ExtendedCell> rather than List<Cell>, so we do not need to cast everywhere in the code.

This is a breaking change for coprocessor users, especially that if you implement your own InternalScanner. In general, we can make sure that all the elements in the return List are ExtendedCells, thus Cells, so you are free to cast them to Cells when you want to intercept the results. And all Cells created via CellBuilder are all ExtendedCells, so you are free to cast them to ExtendedCells before adding to the List, or you can cast the List to List<Cell> or even List to add Cells to it.