gui: Avoid redundant tx status updates

In TransactionTablePriv::index, avoid calling
interfaces::Wallet::tryGetTxStatus if the status is up to date as of the most
recent NotifyBlockTip notification.  Store height from the most recent
notification in a new ClientModel::cachedNumBlocks variable in order to check
this.

This avoids floods of IPC traffic from tryGetTxStatus with #10102 when there
are a lot of transactions. It might also make the GUI a little more efficient
even when there is no IPC.

src/qt/clientmodel.h

@@ -56,6 +56,7 @@ public:
 
     //! Return number of connections, default is in- and outbound (total)
     int getNumConnections(unsigned int flags = CONNECTIONS_ALL) const;
+    int getNumBlocks() const;
     int getHeaderTipHeight() const;
     int64_t getHeaderTipTime() const;
 
@@ -73,9 +74,10 @@ public:
 
     bool getProxyInfo(std::string& ip_port) const;
 
-    // caches for the best header
+    // caches for the best header, number of blocks
     mutable std::atomic<int> cachedBestHeaderHeight;
     mutable std::atomic<int64_t> cachedBestHeaderTime;
+    mutable std::atomic<int> m_cached_num_blocks{-1};
 
 private:
     interfaces::Node& m_node;