Add server push functionality

[finnje]

I've noticed when I run inn fetch <interface> that communication between the two associated peers doesn't work until I run inn fetch <interface> on both peers. That's a little cumbersome and would be nice to handle that automatically via the innernet-server.

[mcginty]

Unfortunately there's no way to remotely trigger an inn fetch on other peers from the server, since there's no push mechanism (clients poll the server, by default, every 60 seconds). You can always override the frequency with systemctl edit though, we do that often.

I'm going to rename this issue to "Add server push functionality" since I think that's what you're asking for here, where clients would establish a long-running connection with the server and receive pushed state updates instead of polling for them.

[finnje]

Sounds good! Thanks for the tip on editing the default poll time, that should be a decent workaround for now.