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/mount/wfs_filer_client.go

@@ -49,3 +49,7 @@ func (wfs *WFS) AdjustedUrl(location *filer_pb.Location) string {
 	}
 	return location.Url
 }
+
+func (wfs *WFS) GetDataCenter() string {
+	return wfs.option.DataCenter
+}