security: Split KeyChain into signing (KeyChain) and verification (Verifier) interfaces

This split removes the need for IdentityManager.

Also in this commit: Make verifySignature methods a set of static methods of Verifier.

Change-Id: Iea1c4353857a21417b2dcd1f91ba7013995d1459
diff --git a/include/ndn-cpp/security/policy/validation-request.hpp b/include/ndn-cpp/security/policy/validation-request.hpp
index 78f2d6c..69a1079 100644
--- a/include/ndn-cpp/security/policy/validation-request.hpp
+++ b/include/ndn-cpp/security/policy/validation-request.hpp
@@ -9,7 +9,7 @@
 #ifndef NDN_VALIDATION_REQUEST_HPP
 #define NDN_VALIDATION_REQUEST_HPP
 
-#include "../key-chain.hpp"
+#include "../../interest.hpp"
 
 namespace ndn {