return all live brokers in TopicMetadataResponse

Currently, we only return the brokers that have assigned replicas for a topic in TopicMetadataResponse. The new producer will use those brokers for refreshing metadata. Now suppose that we stop all those brokers, copy all local data to some new hosts and then restart those hosts (with the original broker id). There is no way for the new producer to automatically get the information about the new brokers since all old brokers are gone.