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-forwarder-status.hpp b/src/management/nfd-forwarder-status.hpp
index 7dcf2a4..9ac58c4 100644
--- a/src/management/nfd-forwarder-status.hpp
+++ b/src/management/nfd-forwarder-status.hpp
@@ -39,12 +39,12 @@
 class ForwarderStatus
 {
 public:
-  class Error : public Tlv::Error
+  class Error : public tlv::Error
   {
   public:
     explicit
     Error(const std::string& what)
-      : Tlv::Error(what)
+      : tlv::Error(what)
     {
     }
   };
@@ -314,7 +314,7 @@
                                                 m_nfdVersion);
 
   totalLength += encoder.prependVarNumber(totalLength);
-  totalLength += encoder.prependVarNumber(Tlv::Content);
+  totalLength += encoder.prependVarNumber(tlv::Content);
   return totalLength;
 }
 
@@ -337,7 +337,7 @@
 inline void
 ForwarderStatus::wireDecode(const Block& block)
 {
-  if (block.type() != Tlv::Content) {
+  if (block.type() != tlv::Content) {
     throw Error("expecting Content block for Status payload");
   }
   m_wire = block;