update pipeline to call teardown of output workers when :workers > 1

fixes https://github.com/elasticsearch/logstash/issues/2178
you're right, ideally this would be managed by the output plugin itself. But, if I am not mistaken, this would mean that if someone writes their own teardown (like in ES-output) they would have to first call super's teardown fn as to preserve the correct behavior. I think it is better to abstract these details away so that it is more difficult to make mistakes when writing new plugins.

If I am wrong about that, let me know!