[Foundation] Don't dispose the CancellationTokenSource in NSUrlSessionHandler's InflightData. Fixes #11799.

[rolfbjarne]

There's a random ObjectDisposedException occurring, which seems to have this sequence of events:

1. The request is sent.
2. The request is cancelled.
3. The InflightData instance is removed here:

   xamarin-macios/src/Foundation/NSUrlSessionHandler.cs

   Line 545 in c343732

   RemoveInflightData (dataTask);

   
   3a. The InflightData is disposed:

   xamarin-macios/src/Foundation/NSUrlSessionHandler.cs

   Line 224 in c343732

   data.Dispose ();

   
   3a. The InflightData.CancellationTokenSource instance is disposed:

   xamarin-macios/src/Foundation/NSUrlSessionHandler.cs

   Line 1168 in c343732

   CancellationTokenSource.Dispose ();

4. Data comes in (in the DidReceiveData callback), and SetResponse is called:

   xamarin-macios/src/Foundation/NSUrlSessionHandler.cs

   Line 932 in c343732

   SetResponse (inflight);

   
   4a. The SetResponse method tries to use InflightData.CancellationTokenSource, and that throws an ObjectDisposedException:

   xamarin-macios/src/Foundation/NSUrlSessionHandler.cs

   Line 970 in c343732

   if (inflight.CancellationTokenSource.Token.IsCancellationRequested)

Full stack traces:

InflightData disposed:
   at System.Net.Http.NSUrlSessionHandler.InflightData.Dispose(Boolean disposing) in xamarin-macios/src/Foundation/NSUrlSessionHandler.cs:line 1168
   at System.Net.Http.NSUrlSessionHandler.InflightData.Dispose() in xamarin-macios/src/Foundation/NSUrlSessionHandler.cs:line 1160
   at System.Net.Http.NSUrlSessionHandler.RemoveInflightData(NSUrlSessionTask task, Boolean cancel) in xamarin-macios/src/Foundation/NSUrlSessionHandler.cs:line 224
   at System.Net.Http.NSUrlSessionHandler.<>c__DisplayClass58_0.<SendAsync>b__0() in xamarin-macios/src/Foundation/NSUrlSessionHandler.cs:line 545
   at System.Threading.CancellationToken.<>c.<Register>b__12_0(Object obj)
   at System.Threading.CancellationTokenSource.Invoke(Delegate d, Object state, CancellationTokenSource source)
   at System.Threading.CancellationTokenSource.CallbackNode.<>c.<ExecuteCallback>b__9_0(Object s)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.CancellationTokenSource.CallbackNode.ExecuteCallback()
   at System.Threading.CancellationTokenSource.ExecuteCallbackHandlers(Boolean throwOnFirstException)
   at System.Threading.CancellationTokenSource.NotifyCancellation(Boolean throwOnFirstException)
   at System.Threading.CancellationTokenSource.LinkedNCancellationTokenSource.<>c.<.cctor>b__4_0(Object s)
   at System.Threading.CancellationTokenSource.Invoke(Delegate d, Object state, CancellationTokenSource source)
   at System.Threading.CancellationTokenSource.CallbackNode.ExecuteCallback()
   at System.Threading.CancellationTokenSource.ExecuteCallbackHandlers(Boolean throwOnFirstException)
   at System.Threading.CancellationTokenSource.NotifyCancellation(Boolean throwOnFirstException)
   at System.Threading.CancellationTokenSource.TimerCallback(Object state)
   at System.Threading.TimerQueueTimer.CallCallback(Boolean isThreadPool)
   at System.Threading.TimerQueueTimer.Fire(Boolean isThreadPool)
   at System.Threading.TimerQueue.FireNextTimers()
   at System.Threading.TimerQueue.System.Threading.IThreadPoolWorkItem.Execute()
   at System.Threading.ThreadPoolWorkQueue.DispatchItemWithAutoreleasePool(Object workItem, Thread currentThread)
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading.PortableThreadPool.WorkerThread.WorkerThreadStart()
   at System.Threading.Thread.StartCallback()

ObjectDisposedException:
   *** Terminating app due to uncaught exception 'System.ObjectDisposedException', reason: 'The CancellationTokenSource has been disposed. (System.ObjectDisposedException)
      at System.Net.Http.NSUrlSessionHandler.NSUrlSessionHandlerDelegate.SetResponse(InflightData inflight) in xamarin-macios/src/Foundation/NSUrlSessionHandler.cs:line 970
      at System.Net.Http.NSUrlSessionHandler.NSUrlSessionHandlerDelegate.DidReceiveData(NSUrlSession session, NSUrlSessionDataTask dataTask, NSData data) in xamarin-macios/src/Foundation/NSUrlSessionHandler.cs:line 932
   '
   *** First throw call stack:
   (
      0   CoreFoundation                      0x0000000186e72ccc __exceptionPreprocess + 176
      1   libobjc.A.dylib                     0x000000018695a788 objc_exception_throw + 60
      2   nsurlsessionhandler                 0x0000000106d4562c xamarin_process_managed_exception + 1052
      3   nsurlsessionhandler                 0x00000001071a26c4 _ZL31native_to_managed_trampoline_53P11objc_objectP13objc_selectorPP11_MonoMethodS0_S0_S0_j + 1028
      4   nsurlsessionhandler                 0x00000001071ca8e0 -[System_Net_Http_NSUrlSessionHandler_NSUrlSessionHandlerDelegate URLSession:dataTask:didReceiveData:] + 68
      5   CFNetwork                           0x000000018c0fe28c CFURLCredentialStorageCopyAllCredentials + 61900
      6   libdispatch.dylib                   0x0000000186b6c750 _dispatch_call_block_and_release + 32
      7   libdispatch.dylib                   0x0000000186b6e3e8 _dispatch_client_callout + 20
      8   libdispatch.dylib                   0x0000000186b75a14 _dispatch_lane_serial_drain + 748
      9   libdispatch.dylib                   0x0000000186b76578 _dispatch_lane_invoke + 432
      10  libdispatch.dylib                   0x0000000186b812d0 _dispatch_root_queue_drain_deferred_wlh + 288
      11  libdispatch.dylib                   0x0000000186b80b44 _dispatch_workloop_worker_thread + 404
      12  libsystem_pthread.dylib             0x0000000186d1b00c _pthread_wqthread + 288
      13  libsystem_pthread.dylib             0x0000000186d19d28 start_wqthread + 8

Disposing the CancellationTokenSource would be ideal, but the additional
complexity this would entail makes me question whether it's worth it: the
amount of entry points into this code makes it quite complex to keep track of
what the request has done, what it's doing and what's pending when things can
happen simultaneously on multiple threads at the same time, and keeping the
code thread-safe, and at the same time not accidentally not run into
deadlocks.

So it seems an easy fix would be to just not dispose the
CancellationTokenSource, the GC will eventually do it, even though MSDN
says quite clearly:

Always call Dispose before you release your last reference to the CancellationTokenSource. Otherwise, the resources it is using will not be freed until the garbage collector calls the CancellationTokenSource object's Finalize method.

An additional note is that a network request's resource usage would already be
dwarfing any memory usage by the CancellationTokenSource, so it's unlikely
this will show up in any profiles.

So with this change we won't be disposing the CancellationTokenSource anymore.

Fixes #11799.

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