security: Support ECDSA verification in all related classes.

This commit also addresses some old issues, such as "using namespace std".

Change-Id: I62b8ff97fb2b6f6cfe04b0ad5866bc0797e6e43b
Refs: #1660
diff --git a/src/security/validator.hpp b/src/security/validator.hpp
index b95c5ad..e9bf37e 100644
--- a/src/security/validator.hpp
+++ b/src/security/validator.hpp
@@ -248,6 +248,9 @@
            const OnInterestValidationFailed& onValidationFailed,
            int nSteps);
 
+  static shared_ptr<SignatureWithPublicKey>
+  determineSignatureWithPublicKey(const Signature& signature);
+
   /// Hooks
 
   /**