Add secondary check to prevent housekeeper task from running twice.

This adds an extra check to prevent a HouseKeeperTask from running
more than once simultaneously. This prevents an issue seen in a
misconfigured child task that was queued in concurrent passes. One
instance had not yet finished when the second one started a minute
later, and overwrote a shared pointer, eventually resulting in a
segmentation fault.

Cause spotted by Jonatan Lindblad.

Refs #11902
Add back ArtworkTask enable check... properly.

This adds the check for DailyArtworkUpdates back to ArtworkTask,
following its reversion in ca3a79bf407, but properly runs the
PeriodicHouseKeeperTask check so it does not requeue once per minute.

Refs #11902