Allow other players to continue playing when a player disconnects or crashes.

* Players can rejoin a game in progress in which they started.
* Player numbers are retained and redelivered when the player rejoins.
* Players can locally restart their game as soon as they're ready (again).
* Players are notified of disconnected partners, allowing them to fall back to exploring while waiting for the partner to return.
* Games can no longer be paused (as nobody wanted to implement this).