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/utils/trie/random-policy.h b/utils/trie/random-policy.h
index 1800f95..b5e1d8a 100644
--- a/utils/trie/random-policy.h
+++ b/utils/trie/random-policy.h
@@ -82,6 +82,7 @@
     class type : public policy_container
     {
     public:
+      typedef policy policy; // to get access to get_order methods from outside
       typedef Container parent_trie;
 
       type (Base &base)