use CBlockIndex* insted of uint256 for UpdatedBlockTip signal

- removes mapBlockIndex find operation
- theoretically allows removing the cs_main lock during zqm notification while introducing a new file position lock

src/zmq/zmqnotificationinterface.cpp

@@ -125,12 +125,12 @@ void CZMQNotificationInterface::Shutdown()
     }
 }
 
-void CZMQNotificationInterface::UpdatedBlockTip(const uint256 &hash)
+void CZMQNotificationInterface::UpdatedBlockTip(const CBlockIndex *pindex)
 {
     for (std::list<CZMQAbstractNotifier*>::iterator i = notifiers.begin(); i!=notifiers.end(); )
     {
         CZMQAbstractNotifier *notifier = *i;
-        if (notifier->NotifyBlock(hash))
+        if (notifier->NotifyBlock(pindex))
         {
             i++;
         }