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
Depends on !53 (closed)
Migrated from D1028 (view on Phabricator)