Refactor `Scratchpad` to use PBJ object instead of `SelfSerializable`

`SerializableLong` is used in a single context - it stores last ISS round in ISS scratch pad. `StandardScratchpad` is designed to support any `SelfSerializable` object, it uses only `SerializableLong` and `NodeId`. Both should be migrated to PBJ serialization.

Start by reading StandardScratchpad and tracing its uses of SelfSerializable, SerializableLong, and NodeId, especially the ISS scratch pad storing the last ISS round. Identify the corresponding PBJ serialization entry points and migrate both supported object types; done means StandardScratchpad no longer relies on those SelfSerializable implementations.

Written by the indexing model from the issue text.