Fixed errors and bugs found by valgrind:

- If run with valgrind, mysqltest will now wait longer when syncronizing slave with master
- Ensure that we wait with cleanup() until slave thread has stopped.
- Added signal_thd_deleted() to signal close_connections() that all THD's has been freed.
- Check in handle_fatal_signal() that we don't use variables that has been freed.
- Increased some timeouts when run with --valgrind

Other things:
- Fixed wrong test in one_thread_per_connection_end() if galera is used.
- Removed not needed calls to THD_CHECK_SENTRY() when we are calling 'delete thd'.

mysql-test/mysql-test-run.pl

@@ -5472,6 +5472,12 @@ sub start_mysqltest ($) {
     mtr_add_arg($args, "--max-connections=%d", $opt_max_connections);
   }
 
+  if ( $opt_valgrind )
+  {
+    # Longer timeouts when running with valgrind
+    mtr_add_arg($args, "--wait-longer-for-timeouts");
+  }
+
   if ( $opt_embedded_server )
   {