rand.Source isn't safe for concurrent use.

The previous metric was pubkey1^pubkey2, as specified in the Kademlia
paper. We missed that EC public keys are not uniformly distributed.
Using the hash of the public keys addresses that. It also makes it
a bit harder to generate node IDs that are close to a particular node.

This commit changes the discovery protocol to use the new "v4" endpoint
format, which allows for separate UDP and TCP ports and makes it
possible to discover the UDP address after NAT.

The discovery RPC protocol does not yet distinguish TCP and UDP ports.
But it can't hurt to do so in our internal model.