Constant RENT_EXEMPT_RENT_EPOCH

pub const RENT_EXEMPT_RENT_EPOCH: Epoch = Epoch::MAX; // 18_446_744_073_709_551_615u64

When rent is collected from an exempt account, rent_epoch is set to this value. The idea is to have a fixed, consistent value for rent_epoch for all accounts that do not collect rent. This enables us to get rid of the field completely.