fix issue 52
fix issue 52

keep compact section sorted when input data are not ordered
chrislusf/seaweedfs,chrislusf/seaweedfs,k85169336/seaweedfs,hxiaodon/seaweedfs,k85169336/seaweedfs,hxiaodon/seaweedfs,chrislusf/seaweedfs,chrislusf/seaweedfs