Cleanup Tenderbake code (answering comments)

This MR can be considered as a follow-up of the initial Tenderbake MR (!3738 (merged)).

Its goal is to address most, if not all, remaining open comments, all of them related to modules in lib_protocol. There is indeed a number of improvements pointed out by the reviewer (@mbouaziz) that were not considered strong requirements at the time of the merge, and that were subsequently left out.

This MR is thus a sequence of small commits that address 1 or, at most, 2 comments. The comment, or comments, that the commit addresses is detailed in the commit message.

It's best to review this MR with an additional window opened with the initial Tenderbake MR.