ThreadLocal memory leak in HystrixRequestContext

[petehannam]

This essentially reopens #320.

We (currently) use Tomcat, so are getting Hystrix memory leak errors when it's shutdown or we redeploy.

The issue is that in the shutdown() method, it doesn't clear out the requestVariables field. (I'd be happy to submit a PR, but couldn't see a contributor's guide, so wasn't sure on your process).

For anyone else who's been looking at this, we also had to call Schedulers.shutdown(); in our ServletContextListener, as RxJava holds its own threadpool.

[mattrjacobs]

@petehannam There's no process other than opening a PR. I'd be happy to review one. This hasn't been a priority internally b/c we never attempt shutdown/startup cycles. I understand it would help address problems that you and other have, so it seems worthy of inclusion.

[mattrjacobs]

Closing due to inactivity. Please re-open if there's more to discuss