after called `AsyncFuture.fail`, the future is still running

[haoliang]

thanks for the working implementation. it goes too deep for me. i start to miss 
python's 
[trio](https://trio.readthedocs.io/en/stable/reference-core.html#a-simple-timeout-example):
    
    
    with trio.move_on_after(30):
        result = await do_http_get("https://...";)
        print("result is", result)
    print("with block finished")
    
    
    Run