book checkout -> planet bombardment

[frnsys]

• [x] poll for recent checkout events for this checkout station id, look for one with time newer than the last checkout processed
• [x] get attendee id for recent checkout event, then query that attendee's checkout history
• [x] for a checked out book and attendee
  • [x] show that attendee's planet/regenerate it from their sequence of checked out books and attendee hash
  • [x] spawn the book-object
  • [x] bombard the planet with the book-object
• [x] if a new event is received while animating a bombardment event, halt it and process the new event