code: public static void delete( Key k, Key job_key ) {
      if( k == null ) return;
      Value val = DKV.get(k);
      if( val == null ) return;              // Or just nothing there to delete
      if( !val.isLockable() ) UKV.remove(k); // Simple things being deleted
      else ((Lockable)val.get()).delete(job_key,0.0f); // Lockable being deleted
    }
docs: Write-lock & delete 'k'.  Will fail if 'k' is locked by anybody other than 'job_key'