Delay dispose of CancellationTokenSource when quitting

Quitting in Unity fires a number of events and callbacks. 

`MonoBehaviour.OnApplicationQuit` is called before `Application.wantsToQuit`
which leads to undesired behaviour. `Application.wantsToQuit` can be used to interrupt a quit process, so you can shut down gracefully. If this is utilized, the AgonesSDK has already dispose the cancellation token and future calls will fail. 

Reference: https://docs.unity3d.com/6000.0/Documentation/ScriptReference/Application-wantsToQuit.html

Same solution as in UniRX: neuecc/UniRx#463

sdks/unity/AgonesSdk.cs

@@ -71,6 +71,7 @@ private void Awake()
         {
             String port = Environment.GetEnvironmentVariable("AGONES_SDK_HTTP_PORT");
             sidecarAddress = "http://localhost:" + (port ?? "9358");
+            Application.quitting += OnApplicationQuitting;
         }
 
         private void Start()
@@ -79,10 +80,15 @@ private void Start()
             HealthCheckAsync();
         }
 
-        private void OnApplicationQuit()
+        private void OnApplicationQuitting()
         {
             cancellationTokenSource.Dispose();
         }
+
+        private void OnDestroy()
+        {
+	        Application.quitting -= OnApplicationQuitting;
+        }
         #endregion
 
         #region AgonesRestClient Public Methods