Server: Keep clients' kill, item, and secret counters up to date

Whenever a counter changes, an update is sent to the client.
Fixed|Multiplayer: Server sends total kill, item and secret counts

The client was unaware of the map's total kill, item and secret counts,
as clients do not spawn mobjs when the map is loaded. Now the total
counts are sent, allowing the clients to show correct statistics during
intermission.