Connection encryption between mixnodes (and gateways)

[jstuczyn]

Right now the data exchanged between mixnodes (and also gateways) is sent "in plain". While up to this point that was a no-issue as sphinx packets themselves hide all information. With #359 an adversary observing traffic would be able to infer if given packet is sent in vpn or mix mode. Introducing some encryption between nodes, even something like using 'normal' TLS would completely solve that issue.