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")
5 files changed