Proto/abaab: update slot owner

What

Update the slot owner function for All Bakers Attest. Sometimes the "legacy" mode must be used, so we have two different functions now.

Why

How

Manually testing the MR

I launched all existing tests with the activation level at 0, it broke when trying to validate denunciations. All denunciations were broken before, because they had to rely on this function to determine the faulty delegate. So you can try double attesting.

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