Fetch messages command should fetch from all workspace members that have a connected-account

[Weiko]

Context

The command currently calls a service with a workspace member uuid in the code, we want to fetch all member ids that have a connected account and ideally delegate this to a job for each member.

[charlesBochet]

Make the command idempotent: when you relaunch the command, it should not create duplicates

[Weiko]

Make the command idempotent: when you relaunch the command, it should not create duplicates

done by @bosiraphael