Change RowNumber operator to be completely streaming

Earlier RowNumberLimit was a streaming operator only if there was a
filter on the row_number column that imposed a limit. Change it to
RowNumberOperator so that it streams the data and retain the filter
node.

A row_number window function with order by is handled by the
TopNRowNumber operator. Add a merge node in the distributed plan if
TopNRowNumber is not partitioned.