face: replace EventEmitter usage with Signal

refs #2300

Change-Id: I17d0d65d2c474b17dd9f23f271a6144b0b4fbc07
diff --git a/daemon/face/face.cpp b/daemon/face/face.cpp
index 7d3c1b7..0adf2ed 100644
--- a/daemon/face/face.cpp
+++ b/daemon/face/face.cpp
@@ -36,10 +36,10 @@
   , m_isOnDemand(false)
   , m_isFailed(false)
 {
-  onReceiveInterest += [this](const ndn::Interest&) { ++m_counters.getNInInterests(); };
-  onReceiveData     += [this](const ndn::Data&) {     ++m_counters.getNInDatas(); };
-  onSendInterest    += [this](const ndn::Interest&) { ++m_counters.getNOutInterests(); };
-  onSendData        += [this](const ndn::Data&) {     ++m_counters.getNOutDatas(); };
+  onReceiveInterest.connect([this] (const ndn::Interest&) { ++m_counters.getNInInterests(); });
+  onReceiveData    .connect([this] (const ndn::Data&)     { ++m_counters.getNInDatas(); });
+  onSendInterest   .connect([this] (const ndn::Interest&) { ++m_counters.getNOutInterests(); });
+  onSendData       .connect([this] (const ndn::Data&)     { ++m_counters.getNOutDatas(); });
 }
 
 Face::~Face()
@@ -120,8 +120,6 @@
 
   m_isFailed = true;
   this->onFail(reason);
-
-  this->onFail.clear();
 }
 
 template<typename FaceTraits>