The last n jobs on each Agent should be preserved for posterity. #872

Related to #871 but separate: When an agent is put offline, there is no link to the last job to run on the agent, only an id that is captured in the comment.

The agent should track the last job that ran on it. E.g.: last_job_id.
The last_job_id job should be protected from deletion to preserve the ability to debug.
In fact the last n jobs should be protected from deletion, where n is at least 1.