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/zmq/zmqnotificationinterface.h

@@ -18,10 +18,11 @@ public:
 
     static CZMQNotificationInterface* CreateWithArguments(const std::map<std::string, std::string> &args);
 
+protected:
     bool Initialize();
     void Shutdown();
 
-protected: // CValidationInterface
+    // CValidationInterface
     void SyncTransaction(const CTransaction &tx, const CBlock *pblock);
     void UpdatedBlockTip(const uint256 &newHashTip);