Merge pull request #80 from manuzhang/at_least_once

add multi-topic support to KafkaSpout
Merge pull request #81 from manuzhang/at_least_once

periodically checkpoint messages and timestamp to Kafka
Merge branch 'at_least_once' of https://github.com/manuzhang/gearpump into at_least_once
move throughput metrics to KafkaBolt