Update libkv dependency

vendor/github.com/golang/protobuf/proto/encode.go

@@ -1075,17 +1075,10 @@ func (o *Buffer) enc_map(p *Properties, base structPointer) error {
 
 func (o *Buffer) enc_exts(p *Properties, base structPointer) error {
 	exts := structPointer_Extensions(base, p.field)
-
-	v, mu := exts.extensionsRead()
-	if v == nil {
-		return nil
-	}
-
-	mu.Lock()
-	defer mu.Unlock()
-	if err := encodeExtensionsMap(v); err != nil {
+	if err := encodeExtensions(exts); err != nil {
 		return err
 	}
+	v, _ := exts.extensionsRead()
 
 	return o.enc_map_body(v)
 }

vendor/github.com/golang/protobuf/proto/extensions.go

@@ -154,7 +154,6 @@ type ExtensionDesc struct {
 	Field         int32       // field number
 	Name          string      // fully-qualified name of extension, for text formatting
 	Tag           string      // protobuf tag style
-	Filename      string      // name of the file in which the extension is defined
 }
 
 func (ed *ExtensionDesc) repeated() bool {