Mempool: maintain a map of manager operations indexed by manager

Context

This MR introduces manager_map into the prevalidation state, that maintains a map of all valid operations in the mempool, indexed by their manager. This new map is not used yet, but it will serve to indicate, inside the Operation_conflict error, the fee that an operation would need in order to replace the previously validated operation from the same manager. See #5197 (closed).

Manually testing the MR

This MR is dead code for now. Just check that it doesn't break the CI.

Checklist

• 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). -> Once the introduced code is used to indicate the needed fee inside the Operation_conflict error, it will be tested in tezt.
• [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