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/tests/unit-tests/transport/dummy-face.hpp b/tests/unit-tests/transport/dummy-face.hpp
index fd724b0..faeae55 100644
--- a/tests/unit-tests/transport/dummy-face.hpp
+++ b/tests/unit-tests/transport/dummy-face.hpp
@@ -54,10 +54,10 @@
   virtual void
   send(const Block& wire)
   {
-    if (wire.type() == Tlv::Interest) {
+    if (wire.type() == tlv::Interest) {
       m_sentInterests->push_back(Interest(wire));
     }
-    else if (wire.type() == Tlv::Data) {
+    else if (wire.type() == tlv::Data) {
       m_sentDatas->push_back(Data(wire));
     }
   }