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_parse.cc

@@ -2575,11 +2575,6 @@ void log_slow_statement(THD *thd)
 
 end:
   delete_explain_query(thd->lex);
-  DBUG_EXECUTE_IF("log_slow_statement_end",
-                  if (dbug_user_var_equals_str(thd, "show_explain_probe_query",
-                                               thd->query()))
-                      dbug_serve_apcs(thd, 1);
-                 );
   DBUG_VOID_RETURN;
 }
 
@@ -6074,7 +6069,7 @@ finish:
   }
 
   /* Free tables. Set stage 'closing tables' */
-  close_thread_tables(thd);
+  close_thread_tables_for_query(thd);
 
 
 #ifndef DBUG_OFF
@@ -6223,7 +6218,8 @@ static bool execute_sqlcom_select(THD *thd, TABLE_LIST *all_tables)
         result->remove_offset_limit();
         if (lex->explain_json)
         {
-          lex->explain->print_explain_json(result, lex->analyze_stmt);
+          lex->explain->print_explain_json(result, lex->analyze_stmt,
+                                           false /* is_show_cmd */);
         }
         else
         {