Change: [Network] Update server's NetworkServerGameInfo only when needed

Split the updating in a "static" version that only needs to be called when a new map is loaded or some settings are changed, and a "dynamic" version that updates everything that changes regularly such as the current game date or the number of spectators.

src/network/core/game_info.h

@@ -94,6 +94,7 @@ const char *GetNetworkRevisionString();
 bool IsNetworkCompatibleVersion(const char *other);
 void CheckGameCompatibility(NetworkGameInfo &ngi);
 
+void FillStaticNetworkServerGameInfo();
 const NetworkServerGameInfo *GetCurrentNetworkServerGameInfo();
 
 void DeserializeGRFIdentifier(Packet *p, GRFIdentifier *grf);