MDEV-15889 Semisync ack thread hits an LOCK_plugin assert at shutdown

MDEV-13073 effectively made the master semisync component depending on
the plugin one through instantiation of THD by its Ack thread.
The thread therefore must be closing its resources prior to
plugin_shutdown(), which was not the case.

Fixed with implementing the requirement.

sql/mysqld.cc

@@ -2240,14 +2240,14 @@ void clean_up(bool print_message)
   lex_free();				/* Free some memory */
   item_create_cleanup();
   tdc_start_shutdown();
+#ifdef HAVE_REPLICATION
+  semi_sync_master_deinit();
+#endif
   plugin_shutdown();
   udf_free();
   ha_end();
   if (tc_log)
     tc_log->close();
-#ifdef HAVE_REPLICATION
-  semi_sync_master_deinit();
-#endif
   xid_cache_free();
   tdc_deinit();
   mdl_destroy();