async usage in examples

The sdk provides async interface to all api calls. The example is a simple console application so the main function has to wait for the top level async call to complete. But this is not equivalent to making every api call blocking. Although the top level async task is blocking, it can still contain multiple parallel running async tasks which are non-blocking. In general any function involves network call should be async so that we can call them in parallel for better IO utilization. The login function also falls into the same category.