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1. It was unused.
2. It was broken. Attempting to register a notification listener would have
   registered the notifier itself.
3. We need to switch to some consistent event/notification system anyways so
   we're going to replace this code no matter what.

(found by @pornin)

NATs only care about TCP/UDP and ports. Using multiaddrs here made this library
really hard to work with correctly.

Furthermore, this library doesn't _actually_ support specifying the internal IP
address. However, we'd still _act_ like the internal IP address mattered. This
caused all sorts of mismatches.

Otherwise, we'll try to port-map localhost.

If we use same NAT agent and call for the same permanent mapping
again we get the same mapping, no harm done.

If router dies, we will remap again.
Just pros, no cons.

Some hardware doesn't support UPnP with durations.