- Make Item_XXX::cleanup() clean List<Cached_item> orderby_fields.
  (Items survive across PS re-executions. Cached_item don't, because
   they keep pointers to fix_field'ed items, etc)
- Move List<Cached_item> out into Group_bound_tracker.

Item_func_or_sum.
Implemented method update_used_tables for class Item_findow_func.
Added the flag Item::with_window_func.
Made sure that window functions could be used only in SELECT list
and ORDER BY clause.
Added test cases that checked different illegal placements of
window functions.

Produce a "not supported" error if one attempts to exclude rows

window frames in full accordance with the SQL standard.

Each window name has to be resolved only once.

Don't attempt to do query pushdown when all tables have been optimized
away (tables_list=NULL)>

Catch errors in window frame definitions

with window functions. Added the test case for it.
Also allowed to use aliases for set functions in partition and order lists
that are specified in window functions.

of the type SQL_I_List<ORDER> rather then the objects of this type.
It allows to replace easily one instance of such a list for another.
Besides it will facilitate to compare two lists if they originate from the
same window specification.
In fact any direct assignment for objects of the type SQL_I_List<ORDER>
was not valid.

It is based on the sum() function, thus much of the logic is shared.

Considering that there are 2 counters stored within the function, one
that handles the null value, while the other that handles the divisor
for the avg computation, it is possible to remove the counter from the
Item_sum_avg. I have not removed it in this patch as we may choose to
refactor the whole code into a separate class.
This remains to be dicussed.

1. Item_windowfunc must check the nullness of the field if it returns the
   value from the result_field.
2. The decimal value must come from the window_func instead of the
   result_field when not fetching values from the result_field.

Added a more extensive test case that highlights if these bugs occur.
The 2 select statements check behaviour for decimal and real typed
fields respectively.

This caused Item_windowfunc to return garbage.

Query result had 0 where it should have had NULLs:
- Make Item_window_func::val* functions honor NULL-handling conventions:
  1. set null_value to indicate whether we've returned a NULL value
  2. val_str and val_decimal should return NULL pointer when they're
     returning SQL NULL value.

Fix assertion failure when sending results to network.
- The assert was due to window func returing SQL NULL despite having
  maybe_null=false
- Fixed by settting Item_window_func::maybe_null correctly in fix_fields

Make query pushdown work in the post-refactored code.
This fixes sequence.group_by test.

The counter keeps track of the number of items added to the sum
function. It is increased when we add a value to the sum function and
decreased when it is removed.

This implementation does not deal with the case where removal of
elements from the window frame causes the item to turn to a null value.

- Handle ORDER BY DESC in window definitions.
- Fix an issue in Frame_range_current_row_top

Add support for "RANGE n PRECEDING|FOLLOWING" frame bounds.
- n is currently limited to whatever Item and Item_sum_plus/minus
  can handle (i.e. no DATETIME intervals).
- Didn't check NULL value handling yet.

Also added win_bit.result (result values checked)

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)

Part#1: Frame_n_rows::next_partition() should not assume that the
current table->record[0] points to the first row in the partition.

Since cursor supports move_to() operation, we dont need this.

The results for this test case are blocked by MDEV-9695.

Add testcases for RANGE UNBOUNDED PRECEDING|FOLLOWING

Support RANGE ... CURRENT ROW as frame's first and second bound.