Revert "NSURLSession Reimplementation (#411)"

This reverts commit 07233534e6.

Source/GSTimeoutSource.m

@@ -0,0 +1,76 @@
+#import "GSTimeoutSource.h"
+
+@implementation GSTimeoutSource
+
+- (instancetype) initWithQueue: (dispatch_queue_t)queue
+                       handler: (dispatch_block_t)handler 
+{
+  if (nil != (self = [super init])) 
+    {
+      dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
+      dispatch_source_set_event_handler(timer, handler);
+      dispatch_source_set_cancel_handler(timer, ^{
+        dispatch_release(timer);
+      });
+
+      _timer = timer;
+      _timeoutMs = -1;
+      _isSuspended = YES;
+    }
+  return self;
+}
+
+- (void) dealloc 
+{
+  [self cancel];
+  [super dealloc];
+}
+
+- (NSInteger) timeout
+{
+  return _timeoutMs;
+}
+
+- (void) setTimeout: (NSInteger)timeoutMs
+{
+  if (timeoutMs >= 0)
+    {
+      _timeoutMs = timeoutMs;
+
+      dispatch_source_set_timer(_timer,
+        dispatch_time(DISPATCH_TIME_NOW, timeoutMs * NSEC_PER_MSEC),
+        DISPATCH_TIME_FOREVER,  // don't repeat
+        timeoutMs * 0.05);      // 5% leeway
+
+      if (_isSuspended)
+        {
+          _isSuspended = NO;
+          dispatch_resume(_timer);
+        }
+    }
+  else
+  {
+    [self suspend];
+  }
+}
+
+- (void)suspend
+{
+  if (!_isSuspended)
+    {
+      _isSuspended = YES;
+      _timeoutMs = -1;
+      dispatch_suspend(_timer);
+    }
+}
+
+- (void) cancel
+{
+  if (_timer)
+    {
+      dispatch_source_cancel(_timer);
+      _timer = NULL; // released in cancel handler
+    }
+}
+
+@end