remove some duplicated code

lib/hk-util.js

@@ -3,7 +3,7 @@
 var HK = module.exports;
 
 const nThen = require('nthen');
-const Once = require("./once");
+const Util = require("./common-util");
 const Meta = require("./metadata");
 const Nacl = require('tweetnacl/nacl-fast');
 
@@ -182,7 +182,7 @@ const computeIndex = function (Env, channelName, cb) {
 
     const ref = {};
 
-    const CB = Once(cb);
+    const CB = Util.once(cb);
 
     const offsetByHash = {};
     let size = 0;