model: New content store variations: support for content freshness

The following content store realizations now fully support freshness set
in ContentObjects:
- ns3::ndn::cs::Freshness::Lru
- ns3::ndn::cs::Freshness::Random
- ns3::ndn::cs::Freshness::Fifo

Example ndn-simple-with-content-freshness demonstrates basics of new
content stores.
diff --git a/apps/ndn-producer.h b/apps/ndn-producer.h
index dc50ea7..dcc9581 100644
--- a/apps/ndn-producer.h
+++ b/apps/ndn-producer.h
@@ -61,6 +61,7 @@
 private:
   NameComponents m_prefix;
   uint32_t m_virtualPayloadSize;
+  Time m_freshness;
 };
 
 } // namespace ndn