Simplify subsystem jobs (#2037)

* Simplify subsystem jobs

This pr simplifies the subsystem jobs interface. Instead of requiring an
extra message that is used to signal that a job should be ended, a job
now ends when the receiver returns `None`. Besides that it changes the
interface to enforce that messages to a job provide a relay parent.

* Drop ToJobTrait

* Remove FromJob

We always convert this message to FromJobCommand anyway.