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/Makefile.am b/Makefile.am
index 79ee8e4..39a0e7e 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -105,11 +105,11 @@
   src/security/certificate/identity-certificate.cpp \
   src/security/certificate/public-key.cpp \
   src/security/identity/basic-identity-storage.cpp \
-  src/security/identity/identity-manager.cpp \
   src/security/identity/identity-storage.cpp \
   src/security/identity/memory-identity-storage.cpp \
   src/security/identity/memory-private-key-storage.cpp \
   src/security/key-chain.cpp \
+  src/security/verifier.cpp \
   src/security/policy/no-verify-policy-manager.cpp \
   src/security/policy/self-verify-policy-manager.cpp \
   src/transport/unix-transport.cpp \