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/util/command-interest-validator.hpp b/src/util/command-interest-validator.hpp
index d132231..689b450 100644
--- a/src/util/command-interest-validator.hpp
+++ b/src/util/command-interest-validator.hpp
@@ -177,7 +177,7 @@
       Signature signature(interestName[POS_SIG_INFO].blockFromValue(),
                           interestName[POS_SIG_VALUE].blockFromValue());
 
-      if (signature.getType() != Tlv::SignatureSha256WithRsa)
+      if (signature.getType() != tlv::SignatureSha256WithRsa)
         return onValidationFailed(interest.shared_from_this(),
                                   "Require SignatureSha256WithRsa");
 
@@ -265,7 +265,7 @@
       return onValidationFailed(interest.shared_from_this(),
                                 "Cannot locate the signing key");
     }
-  catch (Tlv::Error& e)
+  catch (tlv::Error& e)
     {
       return onValidationFailed(interest.shared_from_this(),
                                 "Cannot decode signature related TLVs");