MDEV-7773: Aggregate stored functions

This commit implements aggregate stored functions. The basic idea behind
the feature is:

* Implement a special instruction FETCH GROUP NEXT ROW that will pause
the execution of the stored function. When the instruction is reached,
execution of the initial query resumes "as if" the function returned.
This gives the server the opportunity to advance to the next row in the
result set.

* Stored aggregates behave like regular aggregate functions. The
implementation of thus resides in the class Item_sum_sp. Because it is
an aggregate function, for each new row in the group, the
Item_sum_sp::add() method will be called. This is when execution resumes
and the function does another iteration to "add" one extra element to
the final result.

* When the end of group is reached, val_xxx() method will be called for
the item. This case is handled by another execute step for the stored
function, only with a special flag to force a call to the return
handler. See Item_sum_sp::execute() for details.

To allow this pause and resume semantic, we must preserve the function
context across executions. This is stored in Item_sp::sp_query_arena only for
aggregate stored functions, but has no impact for regular functions.

We also enforce aggregate functions to include the "FETCH GROUP NEXT ROW"
instruction.

Signed-off-by: Vicențiu Ciorbaru <vicentiu@mariadb.org>

mysql-test/r/create_drop_function.result

@@ -3,8 +3,8 @@ CREATE FUNCTION f1(str char(20))
 RETURNS CHAR(100)
 RETURN CONCAT('Hello, ', str, '!');
 SELECT * FROM mysql.proc WHERE name like 'f1';
-db	name	type	specific_name	language	sql_data_access	is_deterministic	security_type	param_list	returns	body	definer	created	modified	sql_mode	comment	character_set_client	collation_connection	db_collation	body_utf8
-test	f1	FUNCTION	f1	SQL	CONTAINS_SQL	NO	DEFINER	str char(20)	char(100) CHARSET latin1	RETURN CONCAT('Hello, ', str, '!')	root@localhost	2014-09-30 08:00:00	2014-09-30 08:00:00	STRICT_TRANS_TABLES,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION		latin1	latin1_swedish_ci	latin1_swedish_ci	RETURN CONCAT('Hello, ', str, '!')
+db	name	type	specific_name	language	sql_data_access	is_deterministic	security_type	param_list	returns	body	definer	created	modified	sql_mode	comment	character_set_client	collation_connection	db_collation	body_utf8	aggregate
+test	f1	FUNCTION	f1	SQL	CONTAINS_SQL	NO	DEFINER	str char(20)	char(100) CHARSET latin1	RETURN CONCAT('Hello, ', str, '!')	root@localhost	2014-09-30 08:00:00	2014-09-30 08:00:00	STRICT_TRANS_TABLES,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION		latin1	latin1_swedish_ci	latin1_swedish_ci	RETURN CONCAT('Hello, ', str, '!')	NONE
 SELECT f1('world');
 f1('world')
 Hello, world!