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/model/cs/content-store-with-stats.cc b/model/cs/content-store-with-stats.cc
index 1e39255..828cbb1 100644
--- a/model/cs/content-store-with-stats.cc
+++ b/model/cs/content-store-with-stats.cc
@@ -63,17 +63,17 @@
 
 #ifdef DOXYGEN
 // /**
-//  * \brief Content Store implementing LRU cache replacement policy
+//  * \brief Content Store with stats implementing LRU cache replacement policy
 //  */
 class Stats::Lru : public ContentStoreWithStats<lru_policy_traits> { };
 
 /**
- * \brief Content Store implementing FIFO cache replacement policy
+ * \brief Content Store with stats implementing FIFO cache replacement policy
  */
 class Stats::Fifo : public ContentStoreWithStats<fifo_policy_traits> { };
 
 /**
- * \brief Content Store implementing Random cache replacement policy
+ * \brief Content Store with stats implementing Random cache replacement policy
  */
 class Stats::Random : public ContentStoreWithStats<random_policy_traits> { };