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_remote_gateway.go

@@ -43,6 +43,10 @@ func (option *RemoteGatewayOptions) AdjustedUrl(location *filer_pb.Location) str
 	return location.Url
 }
 
+func (option *RemoteGatewayOptions) GetDataCenter() string {
+	return ""
+}
+
 var (
 	remoteGatewayOptions RemoteGatewayOptions
 )