Provide centralised storage for ShoppingCart as opposed to cookie

There are several issue currently with the way we store shopping cart:
1. It is cookie based so it can only be accessed by user from specific device which holds the cookie
2. Cart cookie can grow and thus we add to the traffic
3. User cannot resume working with his/her basket from another device.
4. Updates to cart object cause NPE for old cookies

What we need is simple centralised storage (preferable two level: cache and db).
Upon any change to cart it should be put into cache and asynchronously to db.
We also will need mechanism of merging anonymous carts when users log in.