only require userland CLS when needed (#435)

Author: vmarchaud

packages/opencensus-core/src/internal/cls.ts

@@ -27,7 +27,8 @@ const useAsyncHooks: boolean = semver.satisfies(
     process.version, '>=8');  //&&
                               // !!process.env.GCLOUD_TRACE_NEW_CONTEXT;
 
-const cls: typeof CLS = useAsyncHooks ? require('./cls-ah') : CLS;
+const cls: typeof CLS =
+    useAsyncHooks ? require('./cls-ah') : require('continuation-local-storage');
 
 
 const TRACE_NAMESPACE = 'opencensus.io';