valgrind check: do not read potentially deleted variables

let's see if it solves the occational crash in test_highlevel.py
We need save result of shared from this. Then object will be saved until we exits from Goodbye().
```
  void OpcTcpConnection::GoodBye()
   {
     std::shared_ptr<OpcTcpConnection> conn = shared_from_this();
     TcpServer.RemoveClient(conn);
    if (Debug) std::cout << "opc_tcp_async| Good bye." << std::endl;
}
```