add test to demo missing peer records after listen

[yusefnapora]

This shows off what I think is causing #939

The problem is that we're starting our hosts without listen addrs, and apparently the peerstore isn't keeping our signed records since they don't have any addrs. Then, when we call .Listen, it takes ~ 5 seconds for the change to be detected and the new record to be generated.

cc @aarshkshah1992 @raulk @vyzo

[yusefnapora]

The last commits register BasicHost as a network.Notifee so it can get informed when we start or stop listening on an address and call SignalAddressChanges, which causes a new signed record to be created and added to the peerstore right away instead of having to wait for the 5 second ticker to detect the change.

It's still a little racy - on my laptop I only have to sleep for a millisecond after listening before I can get the new record out of the peerstore, but that's not quite long enough on Travis, so I bumped it to 20ms in the test. Still better than five seconds though :)

[Stebalien]

We could get rid of that race by waiting, but that's likely more trouble than it's worth. Instead, we should just fix the panic.