Add retries for refreshMetadata

When broker(s) recovery from shutdown, it will immediately come into zookeeper, which will result in a `brokersChanged` event in kafka-node. When kafka-node client get the event, it will refresh the metadata, but it always need several seconds to wait for the brokers actually be available, so we add a retry once per second if it fail to refresh, so far the ma retry time is 10. Related to issue #60.