model: Another set of refactoring/renaming to make code compile (not tested yet)

Refs #1005 (http://redmine.named-data.net/)
diff --git a/model/cs/custom-policies/freshness-policy.h b/model/cs/custom-policies/freshness-policy.h
index 5fa2a34..b85635e 100644
--- a/model/cs/custom-policies/freshness-policy.h
+++ b/model/cs/custom-policies/freshness-policy.h
@@ -103,7 +103,7 @@
       insert (typename parent_trie::iterator item)
       {
         // get_time (item) = Simulator::Now ();
-        Time freshness = item->payload ()->GetHeader ()->GetFreshness ();
+        Time freshness = item->payload ()->GetData ()->GetFreshness ();
         if (!freshness.IsZero ())
           {
             get_freshness (item) = Simulator::Now () + freshness;
@@ -125,7 +125,7 @@
       inline void
       erase (typename parent_trie::iterator item)
       {
-        if (!item->payload ()->GetHeader ()->GetFreshness ().IsZero ())
+        if (!item->payload ()->GetData ()->GetFreshness ().IsZero ())
           {
             // erase only if freshness is non zero (otherwise an item is not in the policy
             policy_container::erase (policy_container::s_iterator_to (*item));