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_select.cc

@@ -12834,7 +12834,14 @@ end:
 static JOIN_TAB *next_breadth_first_tab(JOIN_TAB *first_top_tab,
                                         uint n_top_tabs_count, JOIN_TAB *tab)
 {
-  n_top_tabs_count += tab->join->aggr_tables;
+  /*
+    tab->join == NULL means that we're performing JOIN::cleanup()
+    after a raised error: on EOM, or on an attempt to create a temporary table
+    with a column of a non allowed data type, such as SYS_REFCURSOR.
+  */
+  DBUG_ASSERT(tab->join || current_thd->is_error());
+  if (tab->join)
+    n_top_tabs_count += tab->join->aggr_tables;
   if (!tab->bush_root_tab)
   {
     /* We're at top level. Get the next top-level tab */
@@ -22087,7 +22094,7 @@ bool Create_tmp_table::add_fields(THD *thd,
                          param->force_copy_fields);
       if (unlikely(!new_field))
       {
-        if (unlikely(thd->is_fatal_error))
+        if (unlikely(thd->is_fatal_error || item->cols() == 1))
           goto err;                             // Got OOM
         continue;                               // Some kind of const item
       }
@@ -22838,6 +22845,11 @@ void Virtual_tmp_table::setup_field_pointers()
       }
     }
     cur_field->reset();
+    /*
+      SYS_REFCURSOR SP variables need NULL as the initial Field value
+      to watch sp_cursor_array_element::m_ref_count properly.
+    */
+    cur_field->set_null();
     field_pos+= cur_field->pack_length();
   }
 }