[rabbitmq] Bump default prefetch setting for consumer up to 256
[rabbitmq] Bump default prefetch setting for consumer up to 256

In practice the number of unacked messages will be the number of
messages "in flight" and not 256. So it does not hurt to increase
the number and get better delivery throughput from RabbitMQ.

Aloomaio/logstash-kafka,joekiller/logstash-kafka,opentable/logstash-kafka,tinglinux/logstash-kafka,tinglinux/logstash-kafka,opentable/logstash-kafka,joekiller/logstash-kafka,Aloomaio/logstash-kafka