Use dispatch_release to release GCD queue in NetworkLinkEvidenceSource

Just a small correction triggered by today's report #246 and my follow-up scan of the related code: While using **CFRelease** on a GCD queue (created with **dispatch_queue_create**) may possibly work as expected -- as least, it did not cause any notable problems on my system and (I assume) on quite a few other systems, for a while. But Apple officially and explicitly requests to use **dispatch_release** to decrement the reference count of any dispatch object, including the GCD queue. Thus let's do it that way.

The info provided with #246 so far was not sufficient for me to find (or make a good guess on -- and then dupe) the actual root cause of that issue. Thus I would not stick my neck out to state that it will be eliminated with this change. But, at least, the change won't make things worse. :)  [That said, interestingly, the crash reported did somehow happen when calling **CFRelease** on the queue.]