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-control-response.hpp b/src/management/nfd-control-response.hpp
index 5b78405..29aa832 100644
--- a/src/management/nfd-control-response.hpp
+++ b/src/management/nfd-control-response.hpp
@@ -38,12 +38,12 @@
 class ControlResponse
 {
 public:
-  class Error : public Tlv::Error
+  class Error : public tlv::Error
   {
   public:
     explicit
     Error(const std::string& what)
-      : Tlv::Error(what)
+      : tlv::Error(what)
     {
     }
   };