Proxy server for split testing

What is the best proxy server to use for split testing?

I need to split traffic to my ecommerce website between different versions of the site in order to test the effectiveness of new features.

I have versions setup as separate urls (separate app deployments) (e.g. web-test1.myapp.com, web-test2.myapp.com).

I'd like to point traffic at one url (e.g. http://ift.tt/1VQNoXy) which would be the proxy and have it split across the backend servers. It has to be sticky, so when a particular backend is selected, it's used for the duration of that session or possibly forever. Need to be able to cycle backends in and out.

I know things like Optimizely exist, but I don't need a tool to create the versions (they're mostly code changes, not purely cosmetic), and I have Google Analytics setup for analytics/measurement, so I don't need an additional reporting tool.

Does anyone know of a simple solution that can do this?