Netcode: allow observers to lag behind the live game.
Observers no longer lag the game for players. There is still some time to serialise the game when sending it to a joining observer, and depending on the chosen 'max lag' the game may stop while observers sufficiently catch up, but this impact too is reduced.
- Make the NetServerTurnManager ignore players marked as 'observers' for the purpose of ending a turn, effectively making it possible for observers to lag without it affecting the players in any way.
- Add a config option (network.observermaxlag) that specifies how many turns behind the live game observers are allowed to be. Default to 10 turns, or 2 seconds, to keep them 'largely live'.
- The controller is not treated as an observer.
- Implement a simple UI to show this delay & allow the game to speed up automatically to try and catch up. This can be deactivated via network.autocatchup.
- Move network options to the renamed 'Network / Lobby' options page.
- Do not debug_warn/crash when receiving commands from the past - instead warn and carry on, to avoid DOS and "coop play" issues.
Differential Revision: https://code.wildfiregames.com/D3737