[2.x] Don't count items, when getting items from all pages

[kgilden]

Currently Page::getItemsOfAllPages() is making an unnecessary call to Page::getItemCount(). That's because AdapterInterface::getItems() requires both offset & limit to be present. By changing the interface to make limit optional, we could retrieve all items by calling Adapter::getItems(0).

Since this breaks BC, let's add it in 2.x.