model: Implementing two more events for forwarding strategy: DidAddFibEntry and WillRemoveFibEntry

Currently, only DidAddFibEntry method is used and only in PerFibLimits
strategy.  Change logic in global routing helper: set per FIB limits only if
forwarding strategy has created a corresponding Limit object.
diff --git a/model/fw/ndn-forwarding-strategy.cc b/model/fw/ndn-forwarding-strategy.cc
index b3e7940..4bab0a1 100644
--- a/model/fw/ndn-forwarding-strategy.cc
+++ b/model/fw/ndn-forwarding-strategy.cc
@@ -564,5 +564,18 @@
   // do nothing here
 }
 
+void
+ForwardingStrategy::DidAddFibEntry (Ptr<fib::Entry> fibEntry)
+{
+  // do nothing here
+}
+
+void
+ForwardingStrategy::WillRemoveFibEntry (Ptr<fib::Entry> fibEntry)
+{
+  // do nothing here
+}
+
+
 } // namespace ndn
 } // namespace ns3