A client can remove a subscription by sending the newline terminated string
unsubscribe(ID) to the server, where ID is the ID of the
subscription to be removed. Recall the when a client makes a subscription,
the server returns a client-unique ID for the subscription. It is this ID
that is used to remove the subscription.
It is the clients responsibility to keep track of the mapping between subscriptions and IDs.
The server will acknowledge the client with a 1 if the unsubscription succeeds and a 0 otherwise.