asyncio client

The update method on the client is blocked until the server finishes. It would be nice if the update method on the client is non-blocking using asyncio. That way multiple models can be updated in parallel using [asyncio.gather](https://docs.python.org/3/library/asyncio-task.html#asyncio.gather).

There is https://github.com/vmagamedov/grpclib which has a asynchronous client.