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/zmqabstractnotifier.cpp

@@ -11,7 +11,7 @@ CZMQAbstractNotifier::~CZMQAbstractNotifier()
     assert(!psocket);
 }
 
-bool CZMQAbstractNotifier::NotifyBlock(const uint256 &/*hash*/)
+bool CZMQAbstractNotifier::NotifyBlock(const CBlockIndex * /*CBlockIndex*/)
 {
     return true;
 }