code: def free(self, request_key):
          
          state = self.get(request_key)
          if not state:
              return
          state.forwardedTo -= 1
          self._clean(state)
docs: Works together with 'mark_as_forwarded' and
          'mark_as_executed' methods.
  
          It makes request to be removed if all replicas request was
          forwarded to freed it and if request executor marked it as executed.