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/test-interest.cpp b/tests/unit-tests/test-interest.cpp
index 82bf8bd..5430bbd 100644
--- a/tests/unit-tests/test-interest.cpp
+++ b/tests/unit-tests/test-interest.cpp
@@ -425,7 +425,7 @@
   const Block& payload = nfd::LocalControlHeader::getPayload(wireBlock);
   BOOST_REQUIRE_NE(&payload, &wireBlock);
 
-  BOOST_CHECK_EQUAL(payload.type(), static_cast<uint32_t>(Tlv::Interest));
+  BOOST_CHECK_EQUAL(payload.type(), static_cast<uint32_t>(tlv::Interest));
   BOOST_CHECK_EQUAL(wireBlock.type(), static_cast<uint32_t>(tlv::nfd::LocalControlHeader));
 
   Interest interest(payload);