- This reworks the CallerReference structure for Route53 health checks in a
  non-backwards compatible way. This means records will create new healthchecks
  for themselves which should be fine.
- Since we're pre 1.0, support has NOT been added to cleanup the old
  healthchecks. That could be done reasonably easy, BUT we'd have to keep that
  around forever. The hope is that the new ref format/usage will prevent this
  problem going forward since enough info exists in the ref to identify things
  fully. :fingers_crossed:
- healthcheck GC is much cleaner and more robust thanks to ^
- overall the healthcheck management code is a bit easier to follow and more
  robust now.