persist consumer group offset

1. use one follower
2. read write consumer group offset

weed/mq/broker/broker_topic_partition_read_write.go

@@ -103,7 +103,7 @@ func (b *MessageQueueBroker) genLogOnDiskReadFunc(t topic.Topic, partition *mq_p
 
 	eachFileFn := func(entry *filer_pb.Entry, eachLogEntryFn log_buffer.EachLogEntryFuncType, starTsNs, stopTsNs int64) (processedTsNs int64, err error) {
 		if len(entry.Content) > 0 {
-			glog.Warningf("this should not happen. unexpected content in %s/%s", partitionDir, entry.Name)
+			// skip .offset files
 			return
 		}
 		var urlStrings []string