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_prepare.cc

@@ -4444,7 +4444,7 @@ bool Prepared_statement::prepare(const char *packet, uint packet_len)
   /* No need to commit statement transaction, it's not started. */
   DBUG_ASSERT(thd->transaction->stmt.is_empty());
 
-  close_thread_tables(thd);
+  close_thread_tables_for_query(thd);
   thd->mdl_context.rollback_to_savepoint(mdl_savepoint);
 
   /*