support LAN/WAN split DHT composition

[willscott]

A more specific case of routing helper that is not currently implemented by this set of utility libraries is the use case where we may want 2 instances of the DHT present.

In a world where a node has both a local network and is connected to the internet, we have a specific instance of composition that we should make easy to support:

• A LAN and WAN dht instance
• If there are peers in the WAN DHT routing table, Put, Provide, etc. requests should be made there. otherwise to LAN.
• Get, FindProviders, FindPeer, etc. requests should pull data from both DHTs and combine them.

type DualDHT struct {
    LAN, WAN *DHT
}

// implementation of `routing.Routing` interface on `*DualDHT`