* 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