public void Remove(ISchemaChangeTracker tracker)
        {
            int i = trackers.FindIndex(x => x.Item1 == tracker);

            if (i >= 0)
            {
                var t = trackers[i];
                trackers.RemoveAt(i);
                t.Item2.Dispose();
            }
        }
        public void Add(ISchemaChangeTracker tracker)
        {
            var subscription = tracker.WhenSchemaNotification.Subscribe(whenSchemaNotification);

            trackers.Add(Tuple.Create(tracker, subscription));
        }