Avoid using sessions for storing ProcessPaymentRequest

[skoshelev]

At the moment we use sessions to store the ProcessPaymentRequest object during the checkout process. But this solution is associated with a number of problems, the main one being the lack of a session for AJAX requests. We encountered this problem at the stage of developing the WEB API, I think other developers have the same problem.
It is better to use generic attributes for these purposes

[ahmadkq]

GenericAttributes table has lot of records and its performance is not good. I suggest to use something else.