Refactor the run loop implementation code into a proper queue abstraction.

ASRunLoopQueue can process a specified number of items per run loop iteration
via a block based api.

AsyncDisplayKit/ASRunLoopQueue.h

@@ -0,0 +1,23 @@
+//
+//  ASRunLoopQueue.h
+//  AsyncDisplayKit
+//
+//  Created by Rahul Malik on 3/7/16.
+//  Copyright © 2016 Facebook. All rights reserved.
+//
+
+#import <Foundation/Foundation.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface ASRunLoopQueue<ObjectType> : NSObject
+
+- (instancetype)initWithRunLoop:(CFRunLoopRef)runloop andHandler:(void(^)(ObjectType dequeuedItem, BOOL isQueueDrained))handlerBlock;
+
+- (void)enqueue:(ObjectType)object;
+
+@property (nonatomic, assign) NSUInteger batchSize;
+
+@end
+
+NS_ASSUME_NONNULL_END