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_select.cc

@@ -26922,6 +26922,7 @@ int print_explain_message_line(select_result_sink *result,
                                ha_rows *rows,
                                const char *message)
 {
+  /* Note: for SHOW EXPLAIN, this is caller thread's THD */
   THD *thd= result->thd;
   MEM_ROOT *mem_root= thd->mem_root;
   Item *item_null= new (mem_root) Item_null(thd);