node: Implement prefix 'unreg' when interest filter is unset

Filter unsetting is useful in some applications that want to terminate
after data has been successfully pushed out to the forwarding daemon.

Change-Id: Iabada0db7d1bd653899941a294abd4c8cd20a80f
diff --git a/include/ndn-cpp/node.hpp b/include/ndn-cpp/node.hpp
index bbf89fd..b5e4e97 100644
--- a/include/ndn-cpp/node.hpp
+++ b/include/ndn-cpp/node.hpp
@@ -330,6 +330,8 @@
   PendingInterestTable pendingInterestTable_;
   RegisteredPrefixTable registeredPrefixTable_;
   Interest ndndIdFetcherInterest_;
+
+  int64_t faceId_; // internal face ID (needed for prefix de-registration)
   Buffer ndndId_;
 };