one thing that I did not like on the new `_updateBusytimes` logic is that it will always be a single `add` or `remove` per event, so I don't really see the point of the hash `{added, removed}` and the arrays.. I would simply do something like `this._addBusytime(event.data)` and `this._removeBusytime(event.data)` or just call it directly like `this._updateBusyCount(event.data, -1)`.