Convert dispatch_queue_t to NSOperationQueue for callbacks.

Mobile Buy SDK/Mobile Buy SDK Tests/BUYIntegrationTest.m

@@ -1303,7 +1303,7 @@
 	NSString *apiKey = [self shouldUseMocks] ? BUYAPIKey_Placeholder : self.apiKey;
 	NSString *appId = [self shouldUseMocks] ? BUYAppId_Placeholder : self.appId;
 	BUYClient *testClient = [[BUYClient alloc] initWithShopDomain:shopDomain apiKey:apiKey appId:appId];
-	testClient.queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
+	testClient.callbackQueue = [NSOperationQueue new];
 	
 	[testClient getShop:^(BUYShop *shop, NSError *error) {
 		BOOL isMainThread = [NSThread isMainThread];

Mobile Buy SDK/Mobile Buy SDK/Data/BUYClient.h

@@ -95,15 +95,14 @@ NS_ASSUME_NONNULL_BEGIN
 @property (nonatomic, strong, nonnull) BUYModelManager *modelManager;
 
 /**
- *  Queue where on which all request operation are executed
+ *  Queue on which all request operation are executed
  */
 @property (nonatomic, strong, readonly, nonnull) NSOperationQueue *requestQueue;
 
 /**
- *  Queue where callbacks will be called
- *  defaults to main queue
+ *  Queue on which network completion callbacks will be executed
  */
-@property (nonatomic, strong, nonnull) dispatch_queue_t queue;
+@property (nonatomic, strong, nonnull) NSOperationQueue *callbackQueue;
 
 /**
  *  The page size for any paged request. This can range from 1-250.  Default is 25

Mobile Buy SDK/Mobile Buy SDK/Data/BUYClient.m

@@ -58,7 +58,7 @@ static NSString * const BUYClientJSONMimeType = @"application/json";
 		_apiKey = apiKey;
 		_appId = appId;
 		_applicationName = [[NSBundle mainBundle] infoDictionary][@"CFBundleName"] ?: @"";
-		_queue = dispatch_get_main_queue();
+		_callbackQueue = [NSOperationQueue mainQueue];
 		_requestQueue = [NSOperationQueue new];
 		_session = [self urlSession];
 		_pageSize = 25;
@@ -174,9 +174,9 @@ static NSString * const BUYClientJSONMimeType = @"application/json";
 	request.HTTPMethod = method;
 	
 	BUYRequestOperation *operation = [[BUYRequestOperation alloc] initWithSession:self.session request:request payload:object completion:^(NSDictionary *json, NSURLResponse *response, NSError *error) {
-		dispatch_async(self.queue, ^{
+		[self.callbackQueue addOperationWithBlock:^{
 			completionHandler(json, response, error);
-		});
+		}];
 	}];
 	
 	[self startTask:operation];