libhdfs++: DatanodeConnection::Cancel should not delete the underlying socket

DataNodeConnectionImpl calls reset on the unique_ptr that references the underlying asio::tcp::socket. If this happens after the continuation pipeline checks the cancel state but before asio uses the socket it will segfault because unique_ptr::reset will explicitly change it's value to nullptr.

Cancel should only call shutdown() and close() on the socket but keep the instance of it alive. The socket can probably also be turned into a member of DataNodeConnectionImpl to get rid of the unique pointer and simplify things a bit.