Increased default maxConcurrentOperationCount, fixing issue SDWebImage#527

Author: MrAlek

SDWebImage/SDWebImageDownloader.m

@@ -65,7 +65,7 @@ - (id)init {
     if ((self = [super init])) {
         _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);