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.

mysql-test/main/show_explain_json.test

@@ -478,7 +478,7 @@ drop table if exists t0,t1,t2;
 let $wait_condition= select State='show_explain_trap' from information_schema.processlist where id=$thr2;
 connection con1;
 set @show_explain_probe_query= 'SELECT count(*) - count(*) FROM sys.waits_by_user_by_latency';
-set debug_dbug='+d,log_slow_statement_end';
+set debug_dbug='+d,explain_notify_tables_are_closed';
 
 # Statement guarantees to produce 0 on every run
 send SELECT count(*) - count(*) FROM sys.waits_by_user_by_latency;