Proto: manager counter

Context

Based on !6647 (merged).

This MR makes counter an abstract type instead of on alias of Z.t:

• distinguish from other Z.t and self-document it
• bring the guarantee that it is always non-negative and non-decreasing
• merlin was mixing Z.t with counter

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