Fix broken congestion_interval

Relocated from main logstash repository as I suspect its needed here, not there!

Congestion check seems to run for redis for every single even passing through, or for every batch if using batches. I reproduced the issue on my platform and fixed it. Seems it didn't affect me as much as I was using large batches.

This was reported on the mailing list and here is the fix, which I've tested on my platform.