Fix redis set truncating for message store cache counters.

Currently we do:

```python
keys_removed = yield self.redis.zremrangebyrank(
   self.inbound_key(batch_id), 0, truncate_at)
```

And the Redis docs for `zremrangebyrank` say:

```
Removes all elements in the sorted set stored at key with rank between start and stop.
```