security: Add v2::Validator implementation

Based on the code originally written by Qiuhan Ding

Change-Id: Ib66e24f49d0b6fb2ae21ea1fca7b9ec62ecb753a
Refs: #3289, #1872
diff --git a/src/security/v2/certificate-cache.hpp b/src/security/v2/certificate-cache.hpp
index 4bf9f3c..f649776 100644
--- a/src/security/v2/certificate-cache.hpp
+++ b/src/security/v2/certificate-cache.hpp
@@ -64,13 +64,13 @@
 
   /**
    * @brief Get certificate given key name
-   * @param keyName  Key name for searching the certificate.
+   * @param certPrefix  Certificate prefix for searching the certificate.
    * @return The found certificate, nullptr if not found.
    *
    * @note The returned value may be invalidated after next call to one of find methods.
    */
   const Certificate*
-  find(const Name& keyName);
+  find(const Name& certPrefix) const;
 
   /**
    * @brief Find certificate given interest
@@ -82,7 +82,7 @@
    * @note The returned value may be invalidated after next call to one of find methods.
    */
   const Certificate*
-  find(const Interest& interest);
+  find(const Interest& interest) const;
 
 private:
   class Entry