docs: public T delete_and_lock( Key job_key ) {
      Lockable old =  write_lock(job_key);
      if( old != null ) {
        Log.debug(Log.Tag.Sys.LOCKS,"lock-then-clear "+_key+" by job "+job_key);
        old.delete_impl(new Futures()).blockForPending();
      }
      return (T)this;
    }