face: move isListening()/size() to Channel as pure virtual methods

As a side effect, this ensures that Channel cannot be instantiated.

Change-Id: I875876ddf8fc0fbb04f3e88726c708ece8106702
diff --git a/daemon/face/tcp-channel.cpp b/daemon/face/tcp-channel.cpp
index 5f0f940..978f540 100644
--- a/daemon/face/tcp-channel.cpp
+++ b/daemon/face/tcp-channel.cpp
@@ -70,7 +70,7 @@
                     bool wantLocalFieldsEnabled,
                     const FaceCreatedCallback& onFaceCreated,
                     const FaceCreationFailedCallback& onConnectFailed,
-                    const time::seconds& timeout/* = time::seconds(4)*/)
+                    time::nanoseconds timeout)
 {
   auto it = m_channelFaces.find(remoteEndpoint);
   if (it != m_channelFaces.end()) {
@@ -90,12 +90,6 @@
                                    onFaceCreated, onConnectFailed));
 }
 
-size_t
-TcpChannel::size() const
-{
-  return m_channelFaces.size();
-}
-
 void
 TcpChannel::createFace(ip::tcp::socket&& socket,
                        bool isOnDemand,