MDEV-5654 Server crashes on second installation of daemon_example plugin

daemon plugin: join the thread to make sure it exits before the plugin is unloaded

plugin/daemon_example/daemon_example.cc

@@ -162,6 +162,7 @@ static int daemon_example_plugin_deinit(void *p __attribute__ ((unused)))
   struct tm tm_tmp;
 
   pthread_cancel(con->heartbeat_thread);
+  pthread_join(con->heartbeat_thread, NULL);
 
   localtime_r(&result, &tm_tmp);
   my_snprintf(buffer, sizeof(buffer),