Decouple PLS from MGS.

service/datastore/pls.go

Index: service/datastore/pls.go
diff --git a/service/datastore/pls.go b/service/datastore/pls.go
index e3bb3194fb8a8d57a85f045c4c125a02c9d21306..a323112f3b01602ea81cecbb0f65418765b29745 100644
--- a/service/datastore/pls.go
+++ b/service/datastore/pls.go
@@ -93,7 +93,10 @@ import (
 // methods. So if your GetMeta handles "kind", but you explicitly have a
 // $kind field, the $kind field will take precedence and your GetMeta
 // implementation will not be called for "kind".
-func GetPLS(obj interface{}) PropertyLoadSaver {
+func GetPLS(obj interface{}) interface {
+	PropertyLoadSaver
+	MetaGetterSetter
+} {
 	v := reflect.ValueOf(obj)
 	if v.Kind() != reflect.Ptr || v.Elem().Kind() != reflect.Struct {
 		return &structPLS{c: &structCodec{problem: ErrInvalidEntityType}}
@@ -106,6 +109,13 @@ func GetPLS(obj interface{}) PropertyLoadSaver {
 	return &structPLS{v, c}
 }
 
+func getMGS(obj interface{}) MetaGetterSetter {
+	if mgs, ok := obj.(MetaGetterSetter); ok {
+		return mgs
+	}
+	return GetPLS(obj)
+}
+
 func getCodec(structType reflect.Type) *structCodec {
 	structCodecsMutex.RLock()
 	c, ok := structCodecs[structType]