interface: trigger fewer 'blockchain_updated' notifications

The Qt GUI refreshes all tabs on 'blockchain_updated', which is expensive for very large wallets.
Previously, on every new block, the GUI would get one notification for each connected interface,
now only the fastest interface triggers it.

(was testing with a wallet where refreshing all tabs takes 15 seconds, and 10*15 seconds is
even more annoying...)

electrum/interface.py

@@ -736,24 +736,30 @@ class Interface(Logger):
             if self.tip < constants.net.max_checkpoint():
                 raise GracefulDisconnect('server tip below max checkpoint')
             self._mark_ready()
-            await self._process_header_at_tip()
+            blockchain_updated = await self._process_header_at_tip()
             # header processing done
-            util.trigger_callback('blockchain_updated')
+            if blockchain_updated:
+                util.trigger_callback('blockchain_updated')
             util.trigger_callback('network_updated')
             await self.network.switch_unwanted_fork_interface()
             await self.network.switch_lagging_interface()
 
-    async def _process_header_at_tip(self):
+    async def _process_header_at_tip(self) -> bool:
+        """Returns:
+        False - boring fast-forward: we already have this header as part of this blockchain from another interface,
+        True - new header we didn't have, or reorg
+        """
         height, header = self.tip, self.tip_header
         async with self.network.bhi_lock:
             if self.blockchain.height() >= height and self.blockchain.check_header(header):
                 # another interface amended the blockchain
                 self.logger.info(f"skipping header {height}")
-                return
+                return False
             _, height = await self.step(height, header)
             # in the simple case, height == self.tip+1
             if height <= self.tip:
                 await self.sync_until(height)
+            return True
 
     async def sync_until(self, height, next_height=None):
         if next_height is None: