Non flaky `propose for` command

This MR aims to avoid flakiness when running the propose for command. Indeed, the current implementation can miss some operation if the rest of the command is too fast or too slow (as the monitor mempool rpc is closed as soon as the rest of the command ends). With this MR, we always wait for the mempool answer before going to the next step. This behavior was present in the bake for command, but not back-ported in the propose for.

This was not causing flakiness on master, but in was in the %Non-Blocking RPCs project.

Manually testing the MR

Green CI.

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