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.

Introduce explicitly-defined cursors.
- Rowid_seq_cursor just reads the rowid sequence
- Table_read_cursor also reads rows for the rowids
- Also, we need a kind of cursor that stops at partition boundary?

... into Frame_unbounded_preceding and Frame_unbounded_following

- Item_sum_count::remove() should check if the argument's value is NULL.
- Window Function item must have its Item_window_func::split_sum_func
  called,
- and it must call split_sum_func for aggregate's arguments (see the
  comment near Item_window_func::split_sum_func for explanation why)

Resolved window names.
Checked some constraints for window frames.
Added test cases for window name resolution.

- 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.

- Make queries that use multiple window functions not to leak memory
- Code cleanup in sql_window.cc

These do not have any meaning after MDEV-8646. Their only valid values are
- table_access_tabs= join_tab;
- top_table_access_tabs_count= top_join_tab_count;

Disable the code that attempts to group window functions together
by their PARTITION BY / ORDER BY clauses, because
1. It doesn't work: when I issue a query with just one window function,
  and no indexes on the table, filesort is not invoked at all.
2. It is not possible to check that it works currently.

Add my own code that does invoke filesort() for each window function.
- Hopefully the sort criteria is right
- Debugging shows that filesort operates on {sort_key, rowid} pairs (OK)
- We can read the filesort rowid result in order.

a separate function, JOIN::process_window_functions().

Debug printout should not attempt to print Ref_ptr_array value as %p.

"Distinct" optimization is not used by this query (both in this tree
and in mainline 10.2). Verified this in debugger.

- All parsing problems look like resolved
- Stub performing name resolution of window functions
in simplest queries has been added.

"Re-factor the code for post-join operations".

The patch mainly contains the code ported from mysql-5.6 and
created for two essential architectural changes:
1. WL#5558: Resolve ORDER BY execution method at the optimization stage
2. WL#6071: Inline tmp tables into the nested loops algorithm

The first task was implemented for mysql-5.6 by Ole John Aske.
It allows to make all decisions on ORDER BY operation at the optimization
stage.

The second task implemented for mysql-5.6 by Evgeny Potemkin adds JOIN_TAB
nodes for post-join operations that require temporary tables. It allows
to execute these operations within the nested loops algorithm that used to
be used before this task only for join queries. Besides these task moves
all planning on the execution of these operations from the execution phase
to the optimization phase.

Some other re-factoring changes of mysql-5.6 were pulled in, mainly because
it was easier to pull them in than roll them back. In particular all
changes concerning Ref_ptr_array were incorporated.

The port required some changes in the MariaDB code that concerned the
functionality of EXPLAIN and ANALYZE. This was done mainly by Sergey
Petrunia.