proto: prepare preserved cycle removal - cleanup

Context

This MR contains a few cleanup, some of them following introduction of blocks_preservation_cycles !11300 (merged), and preparing removal of preserved cycle (see !11220 (closed) for full context)

Next step after this MR is !11562 (merged)

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR