* Revert approval-voting subsystem

* Approval voting revert encapsulated within 'ops' module

* use 'get_stored_blocks' to get lower block height

* Fix error message

* Optionally shrink/delete stored blocks range

* range end number is last block number plus 1

* Apply code review suggestions

* Use tristate enum for block range in backend overlay

* Add clarification comment

* Add comments to private struct