MDEV-20034 Add support for the pre-defined weak SYS_REFCURSOR

This patch adds support for SYS_REFCURSOR (a weakly typed cursor)
for both sql_mode=ORACLE and sql_mode=DEFAULT.

Works as a regular stored routine variable, parameter and return value:

- can be passed as an IN parameter to stored functions and procedures
- can be passed as an INOUT and OUT parameter to stored procedures
- can be returned from a stored function

Note, strongly typed REF CURSOR will be added separately.

Note, to maintain dependencies easier, some parts of sql_class.h
and item.h were moved to new header files:

- select_results.h:
  class select_result_sink
  class select_result
  class select_result_interceptor

- sp_cursor.h:
  class sp_cursor_statistics
  class sp_cursor

- sp_rcontext_handler.h
  class Sp_rcontext_handler and its descendants

The implementation consists of the following parts:
- A new class sp_cursor_array deriving from Dynamic_array

- A new class Statement_rcontext which contains data shared
  between sub-statements of a compound statement.
  It has a member m_statement_cursors of the sp_cursor_array data type,
  as well as open cursor counter. THD inherits from Statement_rcontext.

- A new data type handler Type_handler_sys_refcursor in plugins/type_cursor/
  It is designed to store uint16 references -
  positions of the cursor in THD::m_statement_cursors.

- Type_handler_sys_refcursor suppresses some derived numeric features.
  When a SYS_REFCURSOR variable is used as an integer an error is raised.

- A new abstract class sp_instr_fetch_cursor. It's needed to share
  the common code between "OPEN cur" (for static cursors) and
  "OPER cur FOR stmt" (for SYS_REFCURSORs).

- New sp_instr classes:
  * sp_instr_copen_by_ref      - OPEN sys_ref_curor FOR stmt;
  * sp_instr_cfetch_by_ref     - FETCH sys_ref_cursor INTO targets;
  * sp_instr_cclose_by_ref     - CLOSE sys_ref_cursor;
  * sp_instr_destruct_variable - to destruct SYS_REFCURSOR variables when
                                 the execution goes out of the BEGIN..END block
                                 where SYS_REFCURSOR variables are declared.
- New methods in LEX:
  * sp_open_cursor_for_stmt   - handles "OPEN sys_ref_cursor FOR stmt".
  * sp_add_instr_fetch_cursor - "FETCH cur INTO targets" for both
                                static cursors and SYS_REFCURSORs.
  * sp_close - handles "CLOSE cur" both for static cursors and SYS_REFCURSORs.

- Changes in cursor functions to handle both static cursors and SYS_REFCURSORs:
  * Item_func_cursor_isopen
  * Item_func_cursor_found
  * Item_func_cursor_notfound
  * Item_func_cursor_rowcount

- A new system variable @@max_open_cursors - to limit the number
  of cursors (static and SYS_REFCURSORs) opened at the same time.
  Its allowed range is [0-65536], with 50 by default.

- A new virtual method Type_handler::can_return_bool() telling
  if calling item->val_bool() is allowed for Items of this data type,
  or if otherwise the "Illegal parameter for operation" error should be raised
  at fix_fields() time.

- New methods in Sp_rcontext_handler:
  * get_cursor()
  * get_cursor_by_ref()

- A new class Sp_rcontext_handler_statement to handle top level statement
  wide cursors which are shared by all substatements.

- A new virtual method expr_event_handler() in classes Item and Field.
  It's needed to close (and make available for a new OPEN)
  unused THD::m_statement_cursors elements which do not have any references
  any more. It can happen in various moments in time, e.g.
  * after evaluation parameters of an SQL routine
  * after assigning a cursor expression into a SYS_REFCURSOR variable
  * when leaving a BEGIN..END block with SYS_REFCURSOR variables
  * after setting OUT/INOUT routine actual parameters from formal
    parameters.

sql/sql_type.cc

@@ -3416,6 +3416,24 @@ bool Type_handler_bit::
 }
 
 
+/*************************************************************************/
+bool Type_handler_row::Spvar_definition_with_complex_data_types(
+                                                 Spvar_definition *def) const
+{
+  if (def->row_field_definitions())
+  {
+    List_iterator<Spvar_definition> it(*(def->row_field_definitions()));
+    Spvar_definition *member;
+    while ((member= it++))
+    {
+      if (member->type_handler()->is_complex())
+        return true;
+    }
+  }
+  return false;
+}
+
+
 /*************************************************************************/
 bool Type_handler::Key_part_spec_init_primary(Key_part_spec *part,
                                               const Column_definition &def,
@@ -5292,6 +5310,15 @@ bool Type_handler::Item_func_hybrid_field_type_get_date_with_warn(THD *thd,
 }
 
 
+Type_ref_null
+Type_handler::Item_func_hybrid_field_type_val_ref(THD *thd,
+                                             Item_func_hybrid_field_type *item)
+                                                                          const
+{
+  return Type_ref_null();
+}
+
+
 /************************************************************************/
 void Type_handler_decimal_result::Item_get_date(THD *thd, Item *item,
                                                 Temporal::Warn *warn,