utils: Enable use of the simulation time in NFD

Conversion of ndnSIM time to system_clock::time_point, steady_clock::time_point, so that NFD use the simulation time.
diff --git a/helper/ndn-stack-helper.cpp b/helper/ndn-stack-helper.cpp
index bd2413e..5d3dcd2 100644
--- a/helper/ndn-stack-helper.cpp
+++ b/helper/ndn-stack-helper.cpp
@@ -61,6 +61,8 @@
   : m_limitsEnabled(false)
   , m_needSetDefaultRoutes(false)
 {
+  setCustomNdnCxxClocks();
+
   m_ndnFactory.SetTypeId("ns3::ndn::L3Protocol");
   m_strategyFactory.SetTypeId("ns3::ndn::fw::Flooding");
   m_contentStoreFactory.SetTypeId("ns3::ndn::cs::Lru");
@@ -85,6 +87,13 @@
 }
 
 void
+StackHelper::setCustomNdnCxxClocks()
+{
+  ::ndn::time::setCustomClocks(make_shared<ns3::ndn::time::CustomSteadyClock>(),
+                               make_shared<ns3::ndn::time::CustomSystemClock>());
+}
+
+void
 StackHelper::SetStackAttributes(const std::string& attr1, const std::string& value1,
                                 const std::string& attr2, const std::string& value2,
                                 const std::string& attr3, const std::string& value3,
diff --git a/helper/ndn-stack-helper.hpp b/helper/ndn-stack-helper.hpp
index afc43dc..b6b4266 100644
--- a/helper/ndn-stack-helper.hpp
+++ b/helper/ndn-stack-helper.hpp
@@ -303,6 +303,10 @@
   PointToPointNetDeviceCallback(Ptr<Node> node, Ptr<L3Protocol> ndn,
                                 Ptr<NetDevice> netDevice) const;
 
+public:
+  void
+  setCustomNdnCxxClocks();
+
 private:
   StackHelper(const StackHelper&);
   StackHelper&