- Add temporary code: clone_read_record() clones READ_RECORD structure,
  as long as it is used for reading filesort() result that fits into
  memory.
- Add frame bounds for ROWS-type frames. ROWS n PRECEDING|FOLLOWING,
  ROWS UNBOUNDED PRECEDING|FOLLOWING, CURRENT ROW are supported.
- Add Item_sum_count::remove() which allows "streaming" computation
  of COUNT() over a moving frame.