[API] Fix bad device perfect storm

If the worker is dead:
1. On create a device will go into an ERROR state
2. On delete it will get stuck in PENDING_DELETE
  a. But all the backend nodes are removed
  b. Which means that the nodes join in list LB blocks those from being listed
  c. But they still count as quota

Fixed by:
1. Don't delete backend nodes until we remove the device join
2. Set the LB to DELETED even if we can't talk to it
3. Node counts are no longer a join (so they can be 0 if this or similar
   happens again)

Change-Id: I86453ab1c8a070eddf0754dc87daed7b1d4ca45e