SCORU: Check that the number of non committed messages in the inbox stays under the maximum number of messages per commit

Context

Number_of_messages defines a maximum number of messages per commit. If the inbox contains more than this maximum number of uncommitted messages, the rollup is stuck because no more commit can be published.

This MR fixes this issue by tracking the number of messages during the commitment period (currently twenty blocks), refusing to add more messages in the inbox if the maximum number of messages per commit is reached.

This MR also includes several minor improvements of the code quality.

Manually testing the MR

dune exec tezt/tests/main.exe -- --file sc_rollup.ml inbox node

should fail right after the commit introducing the tests and should succeed after the application of the entire commit set.

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).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR