[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
do we want to get rid of fix_due_date? it's true that validations will now catch this, but if we leave this we'll need to fix it in regards to hard/soft reservations in #573.