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-rib-entry.cpp b/src/management/nfd-rib-entry.cpp
index d075cc6..f97fa66 100644
--- a/src/management/nfd-rib-entry.cpp
+++ b/src/management/nfd-rib-entry.cpp
@@ -266,7 +266,7 @@
 
   Block::element_const_iterator val = m_wire.elements_begin();
 
-  if (val != m_wire.elements_end() && val->type() == Tlv::Name) {
+  if (val != m_wire.elements_end() && val->type() == tlv::Name) {
     m_prefix.wireDecode(*val);
     ++val;
   }