FTR: Game End Signal

[proof88]

Currently only a hard coded frag limit ends the game which is monitored constantly on client side as well!
This is no good for server-configured frag limit and/or time limit so we need to introduce a new server -> clients message type to explicitly signal end of game, this way clients wont need to monitor this at all.

Update: note that since v0.2.3 clients receive server config so they could still properly detect game end even if frag limit is not hardcoded. However, with server-configured time limit, it would be more difficult to properly detect game end for clients, so I think it is better if we explicitly introduce this new message type.
Also, from theory point of view, the proper way of syncing clients with server is adding this new message. Otherwise we are just suspecting that clients really detect game end at the same time as server does.
Also in the future other more sophisticated game end goals might be introduced, overall it is better if only server is managing that since server is the single source of truth.

[proof88]

Done for v0.2.4.