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-regex.cpp b/src/security/validator-regex.cpp
index 97f796c..194aaf1 100644
--- a/src/security/validator-regex.cpp
+++ b/src/security/validator-regex.cpp
@@ -161,7 +161,7 @@
                   return;
                 }
             }
-          catch (Tlv::Error& e)
+          catch (tlv::Error& e)
             {
               return onValidationFailed(data.shared_from_this(),
                                         "Cannot decode signature");