Add new method GetItemsAndFlush

We are facing a situation where we need to flush the entire cache while capturing a snapshot of its exact state before doing so.

Using the Itemsfunction followed by Flush is not concurrent safe because a Set operation could occur between the two calls, resulting in the Flush call mistakenly deleting newly added items.

A workaround would be to use a mutex that prevents writes while these two sequential calls are being made. However, it is much simpler and more efficient to use the library's existing mutex.