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.hpp b/src/management/nfd-rib-entry.hpp
index 9ea84c2..738b8f5 100644
--- a/src/management/nfd-rib-entry.hpp
+++ b/src/management/nfd-rib-entry.hpp
@@ -52,11 +52,11 @@
 class Route
 {
 public:
-  class Error : public Tlv::Error
+  class Error : public tlv::Error
   {
   public:
     explicit
-    Error(const std::string& what) : Tlv::Error(what)
+    Error(const std::string& what) : tlv::Error(what)
     {
     }
   };
@@ -187,10 +187,10 @@
 class RibEntry
 {
 public:
-  class Error : public Tlv::Error
+  class Error : public tlv::Error
   {
   public:
-    Error(const std::string& what) : Tlv::Error(what)
+    Error(const std::string& what) : tlv::Error(what)
     {
     }
   };