[Multiplayer] Send heartbeats to dpmaster

[RobertBeckebans]

Instead of relying on the Steam API to connect players directly build a available server list like with vanilla Doom 3 or Quake 3.
Each server regardless if it is dedicated or not should send heartbeats to the master server and each player should be able to see that list on the website or through some external tool and should be able to join the server using the +connect ip command.