Refactor replica state & various cleanups

[Hoverbear]

I'm moving append() back to accepting Ts. Behind the scenes we'll bincode them. This is reasonable because we need to use bincode (or some other serialization format) to transmit things like SocketAddrs in the case of a redirect. I don't really see a need for exposing raw [u8] at this time.

[danburkert]

By keeping it as raw bytes it allows the client to specify the serialization format (e.g. capnproto). I would recommend writing a wrapper struct around the client if you want auto serialization through rustc encodable, but by making the lowest level API require it you limit flexibility for all clients.

[Hoverbear]

That's a good point. Let's leave it [u8] then.