Different mutliaddrs is not enough. Nodes may share transports.
NAT port mappings will likely only work on the base IP/TCP port
pair. We go one step further, and require different root (IP)
addrs. Just in case some NATs group by IP. In practice, this is
what we want: use addresses only if hosts that are on different
parts of the network have seen this address.
If the same peer observed the same address twice, it would be
double counted as different observations. This change adds a map
to make sure we're counting each observer once.
This is easily extended to require more than two observations,
but i have not yet encountered NATs for whom this is relevant.
