Simple workaround to fix vcmiserver shutdown procedure

At the moment, vcmilobby *requires* async writes in order to handle
multiple connections with different speeds and at optimal performance,
without hanging if one player is too slow and can't eat all data server
sent to him at once.

However server (and potentially - client) can not handle this mode and
may shutdown either socket or entire asio service too early, before all
writes are performed, leading to weird freeze on ending scenario where
client would not receive notifications about end of game.

lib/network/NetworkInterface.h

@@ -17,6 +17,7 @@ class DLL_LINKAGE INetworkConnection : boost::noncopyable
 public:
 	virtual ~INetworkConnection() = default;
 	virtual void sendPacket(const std::vector<std::byte> & message) = 0;
+	virtual void setAsyncWritesEnabled(bool on) = 0;
 	virtual void close() = 0;
 };