Ensure race condition doesn't save invalid id no.

* the dumb_repmgr_id search looks for the highst id number in use
  but if multiple nodes are coming up, they will get the same
  number. The search saves this id and in case of a conflict it
  will never update and the standby clone cmd will fail every time

* use a specific status set after successful slaving as the
  test for whether we should search for highest node number