Refactor swh/scheduler/updater as well

• use the same config system as the main backend, with the same conventions,
• do not make SchedulerUpdaterBackend inherit from DbBackend, use a simple association, similar to the SchedulerBackend class,
• refactor the ghtorrent the same (explicit config),
• same for the updater writer
• move their main functions in cli.py
• adapt tests accordingly