Replace Scratchpad with something simplier and not dependent on SelfSerializable

Scratchpad is a super complicated class, which is used only to store a single long value between the runs and is dependent on SelfSerializable, which we want to get rid of. Some simplier approach should be designed.

* We can keep most of the logic, but use PBJ wrappers instead of SelfSerializable
* We can get rid of entire generic Scratchpad class and just preserve that single ISS long as special case somewhere else

To be discussed.