all: rename Tlv namespace to tlv for consistency

This commit renames the Tlv namespace defined in
tlv.hpp to tlv to be more consistent with tlv::security
and tlv::nfd. A Tlv namespace alias is provided for
backwards compatibility.

Change-Id: I21278d6a077afe7776802c3296997d4c1c44c6c6
diff --git a/src/management/nfd-face-event-notification.hpp b/src/management/nfd-face-event-notification.hpp
index 7afb852..a455e57 100644
--- a/src/management/nfd-face-event-notification.hpp
+++ b/src/management/nfd-face-event-notification.hpp
@@ -48,12 +48,12 @@
 class FaceEventNotification : public FaceFlagsTraits<FaceEventNotification>
 {
 public:
-  class Error : public Tlv::Error
+  class Error : public tlv::Error
   {
   public:
     explicit
     Error(const std::string& what)
-      : Tlv::Error(what)
+      : tlv::Error(what)
     {
     }
   };