Cron workers, regardless from how they are schedules, can't have a
context pointing to a user, project or a namespace. Since they don't
run with arguments, they are never scoped to a single resource.

So clear the context, to make sure that every job that gets scheduled
from within a cron worker doesn't inherit it's context.

This also adds a `with_context` helper to the workers to facilitate
scheduling a context for workers that are spun off from a cron worker.