Retrieve messages using before parameter.

async def _retrieve_messages_before_strategy(self, retrieve):
        """Retrieve messages using before parameter."""
        before = self.before.id if self.before else None
        data = await self.logs_from(self.channel.id, retrieve, before=before)
        if len(data):
            if self.limit is not None:
                self.limit -= retrieve
            self.before = Object(id=int(data[-1]['id']))
        return data