reafactor: centralise functions to update status of workflows

[mdonadoni]

Note that the method _update_workflow_status in consumer.py is now being called in rest/utils.py. I think a refactor is needed, in particular:

1. The logic on how to update the status/logs/job progress/job cache of workflows should not be in consumer.py, but in a dedicated module/package
2. There should only be one way (thus, one function) to stop a workflow (and to start/delete/... it), which both updates the database and interacts with k8s as needed. These functions would/should use the ones from (1), and should not be part of rest.

Proper design/planning is needed, though. Should we do it later or as part of this PR?

Originally posted by @mdonadoni in #563 (comment)