Put a new item that has already been at the front of the cache

[chungphb]

To prevent putting a new item that has already been at the front of the cache, check key == _items.front().first && value == _items.front().second before adding the new item and removing the old one.

[chungphb]

It can be applied to all items to be fair. (Instead of removing the old item, just move it to the front of the cache).

It all comes down to whether adding the new item and removing the old one are faster or slower / safer or riskier then checking the new item and moving the old one to the front of the cache.