item_neighbors for new items

**Issue Description**

I am having trouble getting neighbors for new items that I have added. I was wondering if there is a refresh time for user_neighbors/item_neighbors and how to handle new items. In the latest version, it seems that new items can't be handled until data loading. Can this issue be addressed?

**Steps to Reproduce**

- Add new items to the dataset
- Attempt to retrieve neighbors for the new items
- Observe that no neighbors are returned

**Expected Results**
The new items should have neighbors just like other items in the dataset

**Actual Results**
No neighbors are returned for the new items