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-config.cpp b/src/security/validator-config.cpp
index 387cfe0..83b7a62 100644
--- a/src/security/validator-config.cpp
+++ b/src/security/validator-config.cpp
@@ -537,7 +537,7 @@
       return onValidationFailed(interest.shared_from_this(),
                                 "No valid signature");
     }
-  catch (Tlv::Error& e)
+  catch (tlv::Error& e)
     {
       return onValidationFailed(interest.shared_from_this(),
                                 "Cannot decode signature");
@@ -569,7 +569,7 @@
         time::fromUnixTimestamp(
           time::milliseconds(interestName.get(-signed_interest::MIN_LENGTH).toNumber()));
     }
-  catch (Tlv::Error& e)
+  catch (tlv::Error& e)
     {
       return onValidationFailed(interest,
                                 "Cannot decode signature related TLVs");