async cache decorator

def async_cached(cache, key=hashkey):
    def decorator(fn):
        async def wrapper(*args, **kwargs):
            k = key(*args, **kwargs)
            if k not in cache:
                result = await fn(*args, **kwargs)

                if result is None:
                    return result

                cache[k] = result

            return cache[k]
        return wrapper
    return decorator