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/security/validator.cpp b/src/security/validator.cpp
index 2bc9364..de8c311 100644
--- a/src/security/validator.cpp
+++ b/src/security/validator.cpp
@@ -170,7 +170,7 @@
 
       switch (sig.getType())
         {
-        case Tlv::SignatureSha256WithRsa:
+        case tlv::SignatureSha256WithRsa:
           {
             if (key.getKeyType() != KEY_TYPE_RSA)
               return false;
@@ -185,7 +185,7 @@
             return verifier.VerifyMessage(buf, size,
                                           sig.getValue().value(), sig.getValue().value_size());
           }
-        case Tlv::SignatureSha256WithEcdsa:
+        case tlv::SignatureSha256WithEcdsa:
           {
             if (key.getKeyType() != KEY_TYPE_ECDSA)
               return false;