WIP: Give a way for the shell to know the minimum time between blocks

Context

On the shell team, we are exploring several paths to enhance the bootstrap pipeline and one of them is to give access to the shell a way to know the minimum time between blocks. This allows the bootstrap pipeline to reject ill-formed branches a bit quicker than what is done for the moment.

As I said, this is exploratory, but I want it to give it a try.

Checklist

• Provide automatic testing (see the testing guide).
• Add item in the Development Version section of CHANGES.md (only for new features and bug fixes).