MDEV-28201: Server crashes upon SHOW ANALYZE/EXPLAIN FORMAT=JSON

- Describe the lifetime of EXPLAIN data structures in
  sql_explain.h:ExplainDataStructureLifetime.

- Make Item_field::set_field() call set_refers_to_temp_table()
  when it refers to a temp. table.
- Introduce QT_DONT_ACCESS_TMP_TABLES flag for Item::print.
  It directs Item_field::print to not try access its the
  temp table.
- Introduce Explain_query::notify_tables_are_closed()
  and call it right before the query closes its tables.
- Make Explain data stuctures' print_explain_json() methods
  accept "no_tmp_tbl" parameter which means pass
  QT_DONT_ACCESS_TMP_TABLES when printing items.
- Make Show_explain_request::call_in_target_thread() not call
  set_current_thd(). This wasn't needed as the code inside
  lex->print_explain() uses output->thd anyway. output->thd
  refers to the SHOW command's THD object.

sql/sql_base.h

@@ -161,6 +161,7 @@ TABLE_LIST *find_table_in_list(TABLE_LIST *table,
                                const LEX_CSTRING *db_name,
                                const LEX_CSTRING *table_name);
 int close_thread_tables(THD *thd);
+int close_thread_tables_for_query(THD *thd);
 void switch_to_nullable_trigger_fields(List<Item> &items, TABLE *);
 void switch_defaults_to_nullable_trigger_fields(TABLE *table);
 bool fill_record_n_invoke_before_triggers(THD *thd, TABLE *table,