remove singletons

[bettybao1209]

need to add a new msg type GetInstance in LocalNode class like this:

protected override void OnReceive(object message)
        {
            base.OnReceive(message);
            switch (message)
            {
                case Message msg:
                    BroadcastMessage(msg);
                    break;
                case RelayDirectly relay:
                    OnRelayDirectly(relay.Inventory);
                    break;
                case SendDirectly send:
                    OnSendDirectly(send.Inventory);
                    break;
                case GetInstance _:
                    Sender.Tell(this);
                    break;
            }
        }

[erikzhang]

Please check neo-project/neo#2311

[cloud8little]

test passed.