Small update of PIT. Now pit implementation is policy-templated.

Three versions are explicitly instantiated:
- CcnxPitImpl<persistent_policy_traits> ("ns3::CcnxPit")
- CcnxPitImpl<random_policy_traits>     ("ns3::CcnxPitRandom")
- CcnxPitImpl<lru_policy_traits>        ("ns3::CcnxPitLru")
diff --git a/utils/random-policy.h b/utils/random-policy.h
index 288cd74..2b17fb9 100644
--- a/utils/random-policy.h
+++ b/utils/random-policy.h
@@ -98,7 +98,7 @@
       {
         get_order (item) = u_rand.GetValue ();
 
-        if (policy_container::size () >= max_size_)
+        if (max_size_ != 0 && policy_container::size () >= max_size_)
           {
             if (MemberHookLess<Container>() (*item, *policy_container::begin ()))
               {