Remove some usage of Check_level_instant_set and Sql_mode_save

The reason for the removal are:
- Generates more code
  - Storing and retreving THD
  - Causes extra code and daata to be generated to handle possible throw
    exceptions (which never happens in MariaDB code)
- Uses more stack space

Other things:
- Changed convert_const_to_int() to use item->save_in_field_no_warnings(),
  which made the code shorter and simpler.
- Removed not needed code in Sp_handler::sp_create_routine()
- Added thd as argument to store_key.copy() to make function simpler
- Added thd as argument to some subselect* constructor that inherites
  from Item_subselect.

sql/sql_table.cc

@@ -9712,10 +9712,12 @@ do_continue:;
         Set the truncated column values of thd as warning
         for alter table.
       */
-      Check_level_instant_set check_level_save(thd, CHECK_FIELD_WARN);
+      enum_check_fields org_count_cuted_fields= thd->count_cuted_fields;
+      thd->count_cuted_fields= CHECK_FIELD_WARN;
       int res= mysql_inplace_alter_table(thd, table_list, table, &altered_table,
                                          &ha_alter_info,
                                          &target_mdl_request, &alter_ctx);
+      thd->count_cuted_fields= org_count_cuted_fields;
       my_free(const_cast<uchar*>(frm.str));
 
       if (res)