fix: validate block sync messages

[mversic]

Description

• validate received block sync messages
• optimize FindBlockHeaderByHash
• add start to all_blocks

Linked issue

Closes #{issue_number}

Benefits

Checklist

• [ ] I've read CONTRIBUTING.md
• [ ] I've used the standard signed-off commit format (or will squash just before merging)
• [ ] All applicable CI checks pass (or I promised to make them pass later)
• [ ] (optional) I've written unit tests for the code changes
• [ ] I replied to all comments after code review, marking all implemented changes with thumbs up