Fix token middleware to not trigger CLS init

Rework the token middleware to access current context via
`req.loopbackContext` instead of `loopback.getCurentContext()`.
That way the CLS/AsyncListener machinery is configured only in
applications that are using current context.

server/middleware/token.js

@@ -124,7 +124,7 @@ function token(options) {
     TokenModel.findForRequest(req, options, function(err, token) {
       req.accessToken = token || null;
       rewriteUserLiteral(req, currentUserLiteral);
-      var ctx = loopback.getCurrentContext();
+      var ctx = req.loopbackContext;
       if (ctx) ctx.set('accessToken', token);
       next(err);
     });