Thread safe creation and canceling of download tasks

fixes #364

AsyncDisplayKit/Private/ASBasicImageDownloaderInternal.h

@@ -0,0 +1,22 @@
+/* Copyright (c) 2014-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+#import "ASThread.h"
+
+
+@interface ASBasicImageDownloaderContext : NSObject
+
++ (ASBasicImageDownloaderContext *)contextForURL:(NSURL *)URL;
+
+@property (nonatomic, strong, readonly) NSURL *URL;
+@property (nonatomic, weak) NSURLSessionTask *sessionTask;
+
+- (BOOL)isCancelled;
+- (void)cancel;
+
+@end