Double-close of HedwigSubscriber channel

In HedwigSubscriber, calling unsubscribe():

1. calls closeSubscription() synchronously, which removes the subscriber's channel for the topic
2. asyncSubUnsub uses the publisher's channel to send the UNSUBSCRIBE message, which bypasses the missing subscriber channel
3. (later, when the UNSUBSCRIBE response arrives) UnsubscribeResponseHandler.handleUnsubscribeResponse calls closeSubscription() again, which fails to find the previously closed channel with a warning:

   WARN  - [New I/O client worker #2-2:HedwigSubscriber@759] - Trying to close a subscription when we don't have a subscribe channel cached for topic
   

In the case described above, the double-close is harmless. But it's easy to imagine another situation where a client re-subscribes after unsubscribing, and the async closeSubscription described above causes their new channel to be closed.