IPFS overlay network

Now that we're about to get p2p-circuit support, it would be nice to get an IPFS overlay network. Instead of connecting to all peers directly, nodes (even those not behind NATs) could connect to a subset of their peers through other IPFS nodes. This would:

1. Reduce latencies in some cases as ISPs often pick sub-optimal routes due to peering agreements. That is, the following is sometimes true: `latency(a, b) + latency(b, c) + ẟ < latency(a, c)`.
2. Possibly increase throughput in some cases due to smarter congestion control.
3. Help avoid killing routers by reducing the number of open connections.