Move active players list to one place.

Right now, list of active players is kept in GameController as well as CommandDistributor. One of my previous pull requests (strategy validation) caused the NPE, because player was only removed from CommandDistributor' player list.
This moves the list of active players to CommandDistributor to avoid problems in future.

The change coming with this is that players are reported as removed right when the turn starts, not before it ends.