[Fix Cart Latency] Remove CartReservations!

This PR removes CartReservations entirely from the project. Most calls to CartReservations have been replaced with cart.prepare_all, which builds reservations out of the contents of the cart without needing any interaction with the database
don't we need to change this to `cart.start_date` as well? `set_start_date` is still a method of `Cart` but here we're calling it on the session cart.