Restore state from cache on startup when server is unreachable

[goodhoko]

Innernet client is unable to setup the network interface when it can't reach innernet server on startup. This is a single-point of failure even when other peers are perfectly reachable.

At the same time innernet already caches peer information fetched from the server in /var/lib. We could use this cache on startup when we fail to read state from server.

cc @skywhale @strohel @bschwind

[bschwind]

Related to #296

[strohel]

FWIW this has an interaction with the listen-port being fixed vs. random. If it is fixed, we have higher change of connecting to peers in case of unreachanble server, because peers with the previous (last published) endpoint will still have a chance to find us.

Nothing really actionable, there already is functionality to freeze the listen port, so mentioning just for the record.