Limit SQS message pulls to 10

#### Fix for https://github.com/celery/celery/issues/1746

* Bug reproduced by having a prefetch_multiplier and process
  count that multiply together to become greater than 10. This bug
  is on a per worker basis, and not a per instance basis.
* **Fix**: Anything greater than 10 gets floored to 10.
* **Efficient work around for this hard cap:** Decrease process count per worker
  and add a new worker. Set process count and prefetch multiplier such that
  they multiply to 10.

### Question
It is clear that the original author of this SQS update had the max of 10 in mind when he set the ```max_if_unlimited``` kwarg. **Would it better to use that instead of the hard coded 10?** I stayed away from that since amazon imposes the strict 10 limit, and if someone somehow messes up and passes in the wrong value, this would fix it for them.