Draining attributes in structures

This is so that @cyli can work independently on implementing the algorithm while I work on filling these structures.
I'm sorry, this is probably not a good place for this to go, conceptually.  :|  But I it was the smallest change I could think of for now without introducing another thing that contained the config (like `DesiredLBState`) or something - it'd be nice to move this later, but I wanted to wait on other LB changes and do them all at once.