MDEV-8147: Assertion `m_lock_type == 2' failed in handler::ha_close() during parallel replication

When the slave processes the master restart format_description event,
parallel replication needs to complete any prior events before processing
the restart event (which closes temporary tables and such stuff).

This happens in wait_for_workers_idle(), however it was not waiting long
enough. The wait was using wait_for_prior_commit(), but at that points table
can still be open. This lead to assertion in this case.

So change wait_for_workers_idle() to wait until all worker threads have
reached finish_event_group(), at which point all tables should have been
closed.

sql/sql_class.cc

@@ -6609,6 +6609,7 @@ wait_for_commit::wakeup_subsequent_commits2(int wakeup_error)
     a mutex), so no extra explicit barrier is needed here.
   */
   wakeup_subsequent_commits_running= false;
+  DBUG_EXECUTE_IF("inject_wakeup_subsequent_commits_sleep", my_sleep(21000););
 }