Use low priority queue instead of background for SCNetworkReachabilityGetFlags() (#4587)

AFNetworking/AFNetworkReachabilityManager.m

@@ -227,7 +227,7 @@ static void AFNetworkReachabilityReleaseCallback(const void *info) {
     SCNetworkReachabilitySetCallback(self.networkReachability, AFNetworkReachabilityCallback, &context);
     SCNetworkReachabilityScheduleWithRunLoop(self.networkReachability, CFRunLoopGetMain(), kCFRunLoopCommonModes);
 
-    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0),^{
+    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0),^{
         SCNetworkReachabilityFlags flags;
         if (SCNetworkReachabilityGetFlags(self.networkReachability, &flags)) {
             AFPostReachabilityStatusChange(flags, callback);