Scoru,Proto: skip list index can be a Z.t

Context

Closes #4259 (closed).

Z scales better. We know that the message_counter of a rollup can not be an enormous value, I will anyhow limit the number in #3978 (closed). But a Z.t is safe.

Manually testing the MR

CI.

Checklist

• n/a Document the interface of any function added or modified (see the coding guidelines)
• n/a Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• n/a 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