Handle the result of an iteration in _find.

async def _nodes_found(self, responses):
        """
        Handle the result of an iteration in _find.
        """
        toremove = []
        found_values = []
        for peerid, response in responses.items():
            response = RPCFindResponse(response)
            if not response.happened():
                toremove.append(peerid)
            elif response.has_value():
                found_values.append(response.get_value())
            else:
                peer = self.nearest.get_node(peerid)
                self.nearest_without_value.push(peer)
                self.nearest.push(response.get_node_list())
        self.nearest.remove(toremove)

        if found_values:
            return await self._handle_found_values(found_values)
        if self.nearest.have_contacted_all():
            # not found!
            return None
        return await self.find()