filer prefer volume server in same data center (#3405)

* initial prefer same data center
https://github.com/seaweedfs/seaweedfs/issues/3404

* GetDataCenter

* prefer same data center for ReplicationSource

* GetDataCenterId

* remove glog

weed/command/filer_meta_backup.go

@@ -234,3 +234,7 @@ func (metaBackup *FilerMetaBackupOptions) WithFilerClient(streamingMode bool, fn
 func (metaBackup *FilerMetaBackupOptions) AdjustedUrl(location *filer_pb.Location) string {
 	return location.Url
 }
+
+func (metaBackup *FilerMetaBackupOptions) GetDataCenter() string {
+	return ""
+}