Implement destroy() method for graceful shutdown of MetricsStreamServlet

https://github.com/Netflix/Hystrix/issues/127

This has NOT been tested but is a possible fix to this issue.

@ogafosir I don't have time to validate this but this is probably what is needed. If you can confirm this works then I can merge it in, or if it doesn't go ahead and implement a valid fix and submit a new pull request.