GUACAMOLE-958: Avoid race/deadlock in guacd_timed_client_free()
GUACAMOLE-958: Avoid race/deadlock in guacd_timed_client_free()

glyptodon/guacamole-server,apache/guacamole-server,glyptodon/guacamole-server,apache/guacamole-server,glyptodon/guacamole-server,apache/guacamole-server,mike-jumper/incubator-guacamole-server,mike-jumper/incubator-guacamole-server,mike-jumper/incubator-guacamole-server,mike-jumper/incubator-guacamole-server