Shutting down the publisher should also shut down its spawned tasks

Currently, when the publisher shuts down, its in-progress upload tasks are not interrupted: they either run to completion, or are interrupted just before notifying the publisher reactor of the results: #1244 (comment 2985468)

This ticket is about addressing this point

But maybe letting the upload tasks run uniterrupted when everything else is
shutting down is undesirable. Maybe we can make the publisher watch for the 
shutdown signal too (either directly, or via IPT manager, somehow), and 
replace the upload_for_time_period(...).await with a select! that waits for 
the shutdown signal too.

cc @Diziet @nickm