Discover External IP address via PONG messages

## What was wrong?

Each `PONG` message contains an external IP address that the sender is claiming is our external IP.

This information can be used in combination with some basic heuristics to identify our external ip address.

## How can it be fixed?

We will have to use a heuristic since 1) this information is unverifiable and 2) different peers might have a different view of what our IP address might be.

We should aggregate *votes* for this in a simple service that listens to all the incoming `PONG` messages. This service will benefit from having contextual information about the peer connections such as whether they were dial-out or dial-in, whether they are *trusted* such as bootnode addresses.

Likely only viable once our view of the network contains a sufficiently large number of peers.