Add throttling to LittleProxy.

- Bumped netty version to 4.0.22 for required changes.
- Add GlobalTrafficShapingHandler to DefaultHttpProxyServer for throttling.
I've started working on this. When I run MitmProxyTest with 4.0.14, it passes. 4.0.15 also passes. 4.0.16 gives an SSLHandshakeException. Is this what you're seeing?