face: send Nack after every InterestCallback has responded

refs #4228

Change-Id: I3b1ef58c70d34c2099249216a3600efe01b79f71
diff --git a/src/lp/nack-header.cpp b/src/lp/nack-header.cpp
index 3bbc985..f289c86 100644
--- a/src/lp/nack-header.cpp
+++ b/src/lp/nack-header.cpp
@@ -46,6 +46,19 @@
   return os;
 }
 
+bool
+isLessSevere(lp::NackReason x, lp::NackReason y)
+{
+  if (x == lp::NackReason::NONE) {
+    return false;
+  }
+  if (y == lp::NackReason::NONE) {
+    return true;
+  }
+
+  return static_cast<int>(x) < static_cast<int>(y);
+}
+
 NackHeader::NackHeader()
   : m_reason(NackReason::NONE)
 {