No way to "stop" an individual subscription on a websocket

The subscription model of FireFly allows you to open a single websocket and then [start](https://hyperledger.github.io/firefly/reference/types/subscription.html#using-start-and-ack-explicitly) multiple subscriptions to receive all of their events over that socket.

Currently there is no corresponding "stop" command. The only way to stop receiving events is to close the entire websocket.

It would be convenient to be able to "stop" an individual subscription while leaving the websocket up.