Add throttling to LittleProxy.

- Bumped netty version to 4.0.22 for required changes.
- Add GlobalTrafficShapingHandler to DefaultHttpProxyServer for throttling.
After spending a good part of today in git-bisect, it appears that:
https://github.com/netty/netty/commit/80e6f9adf4ef7eaa273eee8818a4ffd7bb0c5d06
is the commit that breaks SSL in LittleProxy.

I'll dive in further to determine the root cause.