Serialize the `last_request_at` entry as an Integer #2954
Possible solution for #2930.
My first though was of adding support for
Regarding to the testing bits, the existing integration tests felt a bit pointless due the semantics of numbers vs dates - between requests not just the values are the same, but the objects in memory too.
I wanted to cover this better with a unit test for the callback code, but the current state of just passing the blocks to
Pushing the `Time` object inside the session has inconsistencies across different serializers and we should use a more primitive type so we don't need any specific parsing logic for the JSON serializer.