Change: groundwork to allow ServerAddress to use invite codes

Normally TCPConnecter will do a DNS resolving of the connection_string
and connect to it. But for SERVER_ADDRESS_INVITE_CODE this is different:
the Game Coordinator does the "resolving".

This means we need to allow TCPConnecter to not setup a connection
and allow it to be told when a connection has been setup by an external
(to TCPConnecter) part of the code. We do this by telling the (active)
socket for the connection.

This means the rest of the code doesn't need to know the TCPConnecter
is not doing a simple resolve+connect. The rest of the code only
cares the connection is established; not how it was established.

src/network/core/address.h

@@ -185,6 +185,7 @@ public:
  */
 enum ServerAddressType {
 	SERVER_ADDRESS_DIRECT,      ///< Server-address is based on an hostname:port.
+	SERVER_ADDRESS_INVITE_CODE, ///< Server-address is based on an invite code.
 };
 
 /**