util: add literal operators for duration types

Change-Id: I007fa61c80e535c6e4b12e85a32eeeee62fecc3c
Refs: #4468
diff --git a/examples/consumer-with-timer.cpp b/examples/consumer-with-timer.cpp
index a8154f5..76f9853 100644
--- a/examples/consumer-with-timer.cpp
+++ b/examples/consumer-with-timer.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2017 Regents of the University of California.
+ * Copyright (c) 2013-2018 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -48,7 +48,7 @@
   run()
   {
     Interest interest(Name("/example/testApp/randomData"));
-    interest.setInterestLifetime(time::seconds(1));
+    interest.setInterestLifetime(2_s); // 2 seconds
     interest.setMustBeFresh(true);
 
     m_face.expressInterest(interest,
@@ -59,7 +59,7 @@
     std::cout << "Sending " << interest << std::endl;
 
     // Schedule a new event
-    m_scheduler.scheduleEvent(time::seconds(2), [this] { delayedInterest(); });
+    m_scheduler.scheduleEvent(3_s, [this] { delayedInterest(); });
 
     // m_ioService.run() will block until all events finished or m_ioService.stop() is called
     m_ioService.run();
@@ -95,7 +95,7 @@
     std::cout << "One more Interest, delayed by the scheduler" << std::endl;
 
     Interest interest(Name("/example/testApp/randomData"));
-    interest.setInterestLifetime(time::milliseconds(1000));
+    interest.setInterestLifetime(2_s); // 2 seconds
     interest.setMustBeFresh(true);
 
     m_face.expressInterest(interest,