MDEV-14648 Restore fix for MySQL BUG#39053 - UNINSTALL PLUGIN does not allow...

MDEV-14648 Restore fix for MySQL BUG#39053 - UNINSTALL PLUGIN does not allow the storage engine to cleanup open connections

ha_close_connection(): Do invoke the method also for plugins
for which UNINSTALL PLUGIN was deferred due to open connections.

sql/handler.cc

 /* Copyright (c) 2000, 2016, Oracle and/or its affiliates.
-   Copyright (c) 2009, 2016, MariaDB
+   Copyright (c) 2009, 2018, MariaDB Corporation.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -786,7 +786,9 @@ static my_bool closecon_handlerton(THD *thd, plugin_ref plugin,
 */
 void ha_close_connection(THD* thd)
 {
-  plugin_foreach(thd, closecon_handlerton, MYSQL_STORAGE_ENGINE_PLUGIN, 0);
+  plugin_foreach_with_mask(thd, closecon_handlerton,
+			   MYSQL_STORAGE_ENGINE_PLUGIN,
+			   PLUGIN_IS_DELETED|PLUGIN_IS_READY, 0);
 }
 
 static my_bool kill_handlerton(THD *thd, plugin_ref plugin,