Revert "NSURLSession Reimplementation (#411)"

This reverts commit 07233534e6.

Source/GSTaskRegistry.h

@@ -0,0 +1,30 @@
+#ifndef	INCLUDED_GSTASKREGISTRY_H
+#define	INCLUDED_GSTASKREGISTRY_H
+
+#import "common.h"
+
+@class NSArray;
+@class NSURLSessionTask;
+
+/*
+ * This helper class keeps track of all tasks.
+ *
+ * Each `NSURLSession` has a `GSTaskRegistry` for its running tasks. 
+ *
+ * - Note: This must **only** be accessed on the owning session's work queue.
+ */
+@interface GSTaskRegistry : NSObject
+
+- (void ) addTask: (NSURLSessionTask*)task;
+
+- (void) removeTask: (NSURLSessionTask*)task;
+
+- (void) notifyOnTasksCompletion: (void (^)(void))tasksCompletion;
+
+- (NSArray*) allTasks;
+
+- (BOOL) isEmpty;
+
+@end
+
+#endif