Consider making Stream objects

Currently, setting up a stream looks like calling `server.add_stream(...)`, which doesn't return a stream object, or even really create an inspectable stream object. There's another option, of calling `server.get_stream_fn` which sets up a manual stream and returns a callable to add an item to the stream.

I think it would be better if we organized it around `Stream` objects, which kind of feel like Queues. This would simplify the api and make it more recognizable.

Stream Features:
User can always manually `put` to a stream
Stream can be configured to poll a function periodically
User can list or update stream configuration