Add availability statistics service and a simple strategy

Strategy: Simply Choose By Statistics

shadowsocks-csharp/Controller/ShadowsocksController.cs

@@ -25,6 +25,7 @@ namespace Shadowsocks.Controller
         private StrategyManager _strategyManager;
         private PolipoRunner polipoRunner;
         private GFWListUpdater gfwListUpdater;
+        private AvailabilityStatistics _availabilityStatics;
         private bool stopped = false;
 
         private bool _systemProxyIsDirty = false;
@@ -246,6 +247,16 @@ namespace Shadowsocks.Controller
             }
         }
 
+        public void ToggleAvailabilityStatistics(bool enabled)
+        {
+            if (_availabilityStatics != null)
+            {
+                _availabilityStatics.Set(enabled);
+                _config.availabilityStatistics = enabled;
+                SaveConfig(_config);
+            }
+        }
+
         public void SavePACUrl(string pacUrl)
         {
             _config.pacUrl = pacUrl;
@@ -295,6 +306,12 @@ namespace Shadowsocks.Controller
                 _listener.Stop();
             }
 
+            if (_availabilityStatics == null)
+            {
+                _availabilityStatics = new AvailabilityStatistics();
+                _availabilityStatics.UpdateConfiguration(_config);
+            }
+
             // don't put polipoRunner.Start() before pacServer.Stop()
             // or bind will fail when switching bind address from 0.0.0.0 to 127.0.0.1
             // though UseShellExecute is set to true now