Add stop requested/canceled events to pipeline

Motivation: ArchiveBot's pipeline dashboard shows a list of pipelines, but it cannot yet show which pipelines have been `touch STOP`ed: the Seesaw pipeline runner does not provide the pipeline with this information.

This pull request adds events to the pipeline that are triggered when a stop is requested and when a requested stop is canceled.  (The latter appears to only occur via the Warrior interface, but in the future we can wire up e.g. `rm STOP` to also do that.)