Implementing base support for TCP-style window-based limiting on per-FIB-prefix and per-face granularity

Currently, limits are supported only by ndn::fw::Limits forwarding
strategy and only at the very basic (not fully tested) level.
diff --git a/model/pit/ndn-pit-entry-incoming-face.cc b/model/pit/ndn-pit-entry-incoming-face.cc
index 5a7f518..5e3adce 100644
--- a/model/pit/ndn-pit-entry-incoming-face.cc
+++ b/model/pit/ndn-pit-entry-incoming-face.cc
@@ -43,7 +43,7 @@
  * @brie Copy operator
  */
 IncomingFace &
-IncomingFace::operator = (IncomingFace &other)
+IncomingFace::operator = (const IncomingFace &other)
 {
   m_face = other.m_face;
   m_arrivalTime = other.m_arrivalTime;