Scoru: unique inbox / SOL / EOL

Context

Closes #3820 (closed), #3826 (closed)

This merge request changes:

1. The inbox becomes unique, one unique inbox to rule them all. The kernels will need to decide which messages they want to consider
2. Add SOL/EOL in each block

Waiting for !6504 (merged) to be merged

Manually testing the MR

Tests to add:

• Refutation game with messages at or before origination level
• SOL is correctly added (tested implicitly)
• EOL is correctly added (tested implicitly)

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