Refactour out (into a copy for now) the logic of Item_sum_hybrid, to
allow for multiple arguments. It does not contain the comparator
members. The result is the class Item_sum_hybrid_simple.
LEAD and LAG make use of this Item to store previous rows in a chache.
It also helps in specifying the field type. Currently LEAD/LAG do not
support default values.
NTH_VALUE behaves identical to LEAD and LAG, except that the starting
position cursor is placed on the top of the frame instead of the current
row.
