Merge pull request #897 from MrAlek/fix/max-concurrency-count

Increased default maxConcurrentOperationCount, fixes #527

@@ -67,7 +67,7 @@ static NSString *const kCompletedCallbackKey = @"completed";
         _operationClass = [SDWebImageDownloaderOperation class];
         _executionOrder = SDWebImageDownloaderFIFOExecutionOrder;
         _downloadQueue = [NSOperationQueue new];
-        _downloadQueue.maxConcurrentOperationCount = 2;
+        _downloadQueue.maxConcurrentOperationCount = 6;
         _URLCallbacks = [NSMutableDictionary new];
         _HTTPHeaders = [NSMutableDictionary dictionaryWithObject:@"image/webp,image/*;q=0.8" forKey:@"Accept"];
         _barrierQueue = dispatch_queue_create("com.hackemist.SDWebImageDownloaderBarrierQueue", DISPATCH_QUEUE_CONCURRENT);