have the introducer advertise the external IP it sees for a node to other nodes

[daira]

This might reduce the amount of manual configuration needed for tub.location, or work better in cases where a node's external IP is not stable.

It shouldn't introduce security problems because any connecting nodes will check via SSL that they are connecting to the right tubID.

We'd need to think about whether this IP should be added to the advertised location hints, or only be used by default if tub.location isn't configured, and/or when tub.location contains a social marker.