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/face.cpp b/src/face.cpp
index f547f45..e67b9a1 100644
--- a/src/face.cpp
+++ b/src/face.cpp
@@ -384,7 +384,7 @@
 {
   const Block& block = nfd::LocalControlHeader::getPayload(blockFromDaemon);
 
-  if (block.type() == Tlv::Interest)
+  if (block.type() == tlv::Interest)
     {
       shared_ptr<Interest> interest = make_shared<Interest>();
       interest->wireDecode(block);
@@ -393,7 +393,7 @@
 
       m_impl->processInterestFilters(*interest);
     }
-  else if (block.type() == Tlv::Data)
+  else if (block.type() == tlv::Data)
     {
       shared_ptr<Data> data = make_shared<Data>();
       data->wireDecode(block);