Peer to peer ip discovery

Created by: ChrisSchinnerl

So far the only centralized aspect of Sia has been the way a node discovers its public ip address if upnp isn't available. Our workaround was to use myexternalip.com as a fallback but recently they started banning certain ranges of ips and are therefore not reliable. This PR adds a second fallback (that will hopefully replace myexternalip.com completely one day) by using the peers the node is connected to. Once a certain number of peers is available, we query each one of them and if the majority of responses report the same ip, we accept it as our own public ip.