Persist changed world data

When a player makes a change to their world configuration, queue up the world to be persisted - e.g., save the in-memory world data back to the on-disk world file:

• Player updates an item (priority, badge, threshold, etc) - #12 (closed)
• Player requests a notification - #19 (closed)

Consider a top-level interval to persist all "flagged" worlds, and/or a signal catcher to ensure "flagged" worlds are persisted on demand and/or prior to termination.