Part#2: Fix a couple more issues in rows-type frames.
This also has a code cleanup:
- introduce a separate Frame_rows_current_row_(top,bottom). This is
  is a special case which doesn't need its cursor or partition bound check
- Split Frame_n_rows into
  = Frame_n_rows_preceding (this one is now much simpler)
  = Frame_n_rows_following (simpler and works but may need some work still)