Add Object.freeze on private enums

Improve code readability

src/systemData/statusCode.js

@@ -1,9 +1,7 @@
 var ClientMessage = require('../messages/clientMessage');
 var SliceReadStatus = require('../sliceReadStatus');
 
-module.exports = {};
-
-module.exports.convert = function(code) {
+exports.convert = function(code) {
   switch(code) {
     case ClientMessage.ReadStreamEventsCompleted.ReadStreamResult.Success:
       return SliceReadStatus.Success;