Lazy-load CLS module

As soon as CLS module is loaded, the instrumentation/patching of
async-listener is fired. This may cause stack overflows due to promise
instrumentation.

By loading CLS module lazily (only when used for real), we avoid
this kind of problems in applications that are not using current-context
at all.

server/current-context.js

@@ -5,9 +5,21 @@
 
 'use strict';
 
-var cls = require('continuation-local-storage');
 var domain = require('domain');
 
+
+// Require CLS only when using the current context feature.
+// As soon as this require is done, all the instrumentation/patching
+// of async-listener is fired which is not ideal.
+//
+// Some users observed stack overflows due to promise instrumentation
+// and other people have seen similar things:
+//   https://github.com/othiym23/async-listener/issues/57
+// It all goes away when instrumentation is disabled.
+var cls = function() {
+  return require('continuation-local-storage');
+};
+
 var LoopBackContext = module.exports;
 
 /**
@@ -74,7 +86,7 @@ LoopBackContext.createContext = function(scopeName) {
   process.context = process.context || {};
   var ns = process.context[scopeName];
   if (!ns) {
-    ns = cls.createNamespace(scopeName);
+    ns = cls().createNamespace(scopeName);
     process.context[scopeName] = ns;
     // Set up LoopBackContext.getCurrentContext()
     LoopBackContext.getCurrentContext = function() {