Scoru: support executing outbox messages against any cemented commitment saved in context

Context

Owner: @jobjo

Related to #3211 (closed)

Execution of outbox messages is extended to support more than one cemented commitment. An outbox message can be executed against any commitment stored in the context. However, because for the moment we only store the last cemented commitment (tested in the MR) this MR does not introduce any functional change (exception made maybe for gas consumption)

Manually testing the MR

In progress

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