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/sec-rule-specific.cpp b/src/security/sec-rule-specific.cpp
index af89e91..cb7adf4 100644
--- a/src/security/sec-rule-specific.cpp
+++ b/src/security/sec-rule-specific.cpp
@@ -76,7 +76,7 @@
       const Name& signerName = keyLocator.getName();
       return m_signerRegex->match(signerName);
     }
-  catch (Tlv::Error& e)
+  catch (tlv::Error& e)
     {
       return false;
     }