Possible refactor: Make Packet Filtering + Content Routing separate traits about quilkin HOT 2 OPEN

I wonder if this could be solved instead of needing to split filtering from routing, what if instead we had a different way to execute filters to achieve the same effect. Because I think the thing that is interesting about this separation is that you could execute some filtering out of order.

I was thinking in addition to FilterChain, we could have FilterSet (This could also be added to FilterChain but for the sake of discussion assume a separate type), which would allow out-of-order execution of filters, that terminates on the None. The benefit of this, is that we leave it up to the user to decide what they want to execute out-of-order, so that they can configure it to match their business logic. I'm imagining something like the following.

// Executed in-order
FilterChain([
    ValidatePacket,
    // These three are executed out-of-order
    FilterSet([
        RateLimit,
        LoadBalancer,
        BlockList,
    ]),
    // Once the set is completed, the chain continues
    Compress,
    Encrypt,
])

from quilkin.

I think we should revisit this, as routing using filters has already reached some limitations. For example, putting a load balancer filter in front of token router filter can drop the endpoint with the token required to route the packet, and lead to packet loss. Adding more state to routing filters only makes this more complicated and more potential foot guns for users.

Instead we should separate routing from filters entirely, I don't know if we want a Router trait as originally proposed. I think for now we just want to support IP routing and token routing, which could supported in a single enum. So the change would be after the filter chain has run (could also be before if we want to strip the token early), the proxy's Router decides whether to send it to a single IP or checks for a token and routes based on that.

enum Router {
   Ip(SocketAddr),
   LoadBalance(Vec<SocketAddr>),
   Tokens(HashMap<Vec<u8>, Vec<SocketAddr>>),
}

from quilkin.