Added in-game network stats

As requested in #2281

Also wired up the check if clients lag behind the current net frame (labeled as Slowdown). My plan is to expose the network tab to everyone and stats to allies not just spectators once #3135 is merged.

Note this is also useful for #2693 because it exposes the IP to everyone (not just the server and not just hidden in a log file) which can then be traced to identify trolls and get them banned for the next round.