Burrow proxy server

This will allow clients to connect to one server (the proxy) and let it distribute load to one or more backing queue servers. This is used for both HA and spreading load. This will be implemented as a backend module and will perform two-step consistent hashing on the account and queue/message.

For the summit discussion (if we have one), it would be great to have some of the Swift developers look over this and discuss, since it is similar to the distributed hashing they perform.