[FIXES #1777] Make accessing pendingLaunches lock free

- My objection to #1777 is that it is introducing more locking. The ultimate aim is to make
  node provisioning and queue maintenance completely lock free. Thus adding more locks than
  is strictly required is a bad plan.
- We cannot completely remove provisioningLock at this time as it is used to ensure that
  only one provisioning activity can take place for a given label at a time.
- This change ensures that the provisioningLock is only used by the NodeProvisioner for
  its internal thread guarantees and is thus no-longer used accidentally by any
  external threads