Unsubscribing from characteristic causes disconnection on Darwin BT LE

If a central that has subscribed (notifications or indications) to a characteristic unsubscribes from any of them, the darwin backend disconnects the server. This shouldn't be the case; the same central may have more subscriptions and there may be more centrals too.

The root issue is that unsubscription shouldn't be interpreted as a disconnection (even if it was the last subscription on the server, the central(s) may still be connected and running).