Fix exception regarding modification after enumerator

Author: FrankPetrilli

Src/SocketIoClientDotNet.net45/Client/Manager.cs

@@ -397,7 +397,9 @@ private void Cleanup()
         {
             foreach (var sub in Subs)
             {
-                sub.Destroy();
+                lock (Subs) {
+                    sub.Destroy();
+                }
             }
         }