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/main.cpp

@@ -2707,7 +2707,7 @@ bool ActivateBestChain(CValidationState &state, CBlock *pblock) {
                         pnode->PushInventory(CInv(MSG_BLOCK, hashNewTip));
             }
             // Notify external listeners about the new tip.
-            GetMainSignals().UpdatedBlockTip(hashNewTip);
+            GetMainSignals().UpdatedBlockTip(pindexNewTip);
             uiInterface.NotifyBlockTip(hashNewTip);
         }
     } while(pindexMostWork != chainActive.Tip());