Job system should clean up jobs that are "dead in the water"

[einari]

Add functionality to JobsManager that cleans up jobs that are typically stuck in preparation and has no steps and was started before a given time threshold (configurable).

We should also add a TimeStamp on the JobState for when it was created - this time stamp is the one we should use as time threshold.

The goal is not to clean up jobs that are possibly just starting up. So we add a time threshold to distinguish from these. E.g. jobs that hasn't started in an hour (on preparing steps state) are candidates. If they don't then have any steps or any progress, they can be be cleared out.

The JobsManager should do this on startup, but also have a scavenger "job" - basically a timer that runs on a regular cadence (e.g. every hour - configurable).