Fix ZMQ Notification initialization and shutdown

Moves the call Initialize() from init.cpp to CreateWithArguments() and handles the
return value. Moves the call Shutdown() from init.cpp to destructor.
Changes Initialize() and Shutdown() to protected members.

src/init.cpp

@@ -208,7 +208,6 @@ void Shutdown()
 #if ENABLE_ZMQ
     if (pzmqNotificationInterface) {
         UnregisterValidationInterface(pzmqNotificationInterface);
-        pzmqNotificationInterface->Shutdown();
         delete pzmqNotificationInterface;
         pzmqNotificationInterface = NULL;
     }
@@ -1170,7 +1169,6 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
     pzmqNotificationInterface = CZMQNotificationInterface::CreateWithArguments(mapArgs);
 
     if (pzmqNotificationInterface) {
-        pzmqNotificationInterface->Initialize();
         RegisterValidationInterface(pzmqNotificationInterface);
     }
 #endif