Get origin from prefix (#149)

Why I did it
Follow GNMI spec to get origin from prefix.

How I did it
If prefix exists, get origin from prefix.

How to verify it
Run sonic-gnmi unit test.

gnmi_server/server.go

@@ -331,9 +331,11 @@ func (s *Server) Get(ctx context.Context, req *gnmipb.GetRequest) (*gnmipb.GetRe
 	}
 
 	target := ""
+	origin := ""
 	prefix := req.GetPrefix()
 	if prefix != nil {
 		target = prefix.GetTarget()
+		origin = prefix.Origin
 	}
 
 	paths := req.GetPath()
@@ -348,10 +350,11 @@ func (s *Server) Get(ctx context.Context, req *gnmipb.GetRequest) (*gnmipb.GetRe
 	} else if _, ok, _, _ := sdc.IsTargetDb(target); ok {
 		dc, err = sdc.NewDbClient(paths, prefix)
 	} else {
-		origin := ""
-		origin, err = ParseOrigin(paths)
-		if err != nil {
-			return nil, err
+		if origin == "" {
+			origin, err = ParseOrigin(paths)
+			if err != nil {
+				return nil, err
+			}
 		}
 		if check := IsNativeOrigin(origin); check {
 			dc, err = sdc.NewMixedDbClient(paths, prefix, origin, encoding, s.config.ZmqAddress)
@@ -407,6 +410,10 @@ func (s *Server) Set(ctx context.Context, req *gnmipb.SetRequest) (*gnmipb.SetRe
 
 	/* Fetch the prefix. */
 	prefix := req.GetPrefix()
+	origin := ""
+	if prefix != nil {
+		origin = prefix.Origin
+	}
 	extensions := req.GetExtension()
 	encoding := gnmipb.Encoding_JSON_IETF
 
@@ -418,9 +425,11 @@ func (s *Server) Set(ctx context.Context, req *gnmipb.SetRequest) (*gnmipb.SetRe
 	for _, path := range req.GetUpdate() {
 		paths = append(paths, path.GetPath())
 	}
-	origin, err := ParseOrigin(paths)
-	if err != nil {
-		return nil, err
+	if origin == "" {
+		origin, err = ParseOrigin(paths)
+		if err != nil {
+			return nil, err
+		}
 	}
 	if check := IsNativeOrigin(origin); check {
 		if s.config.EnableNativeWrite == false {

sonic_data_client/mixed_db_client.go

@@ -261,8 +261,14 @@ func NewMixedDbClient(paths []*gnmipb.Path, prefix *gnmipb.Path, origin string,
 
 // gnmiFullPath builds the full path from the prefix and path.
 func (c *MixedDbClient) gnmiFullPath(prefix, path *gnmipb.Path) *gnmipb.Path {
-
-	fullPath := &gnmipb.Path{Origin: path.Origin}
+	origin := ""
+	if prefix != nil {
+		origin = prefix.Origin
+	}
+	if origin == "" {
+		origin = path.Origin
+	}
+	fullPath := &gnmipb.Path{Origin: origin}
 	if path.GetElement() != nil {
 		elements := path.GetElement()
 		if prefix != nil {