Handle players disconnecting

[majate]

We need to handle the fact that players can disconnect to the lobby. One possibility is to remove any player that disconnects from a lobby. Another possibility is to mark the player as “offline” and enable the player to join the lobby whenever the connection is found again.

This could possibly be a helpful link in order to implement tracking of player connection and disconnection.

[Gustavrohss]

Players can now reconnect after disconnecting/leaving/etc a game session.