Zebra should auto-discover its own external IP address

[teor2345]

Is your feature request related to a problem? Please describe.

In #1890, we allow users to manually configure an external IP address for Zebra. But we can also detect this address automatically.

Describe the solution you'd like

This ticket depends on #1890. It also uses the unsolicited address advertisements in #1892.

If an IPv4 or IPv6 external address is not configured:

• advertise the internal bind address, if it is a public IP address
• otherwise, track the IPv4 and IPv6 addresses sent by peers in Version.address_recv
• advertise the most popular IPv4 and IPv6 addresses using unsolicited Addr messages (see Zebra should try harder to discover and distribute good peer addresses #1892)
• regularly remove addresses from old handshakes, so that nodes which change addresses learn their new addresses more quickly
  • but always keep at least one IPv4 or IPv6 address

[teor2345]

We probably won't do this any time soon.