interface: log block height of main interface on new block

follow-up 7a7c0f1606
- before ref commit, we logged new headers for every interface
- after ref commit, we logged new header only for fastest interface
- now, we log new header for fastest interface and for main interface
  - useful to see if main interface is very slow

electrum/interface.py

@@ -773,6 +773,8 @@ class Interface(Logger):
             self._mark_ready()
             blockchain_updated = await self._process_header_at_tip()
             # header processing done
+            if self.is_main_server():
+                self.logger.info(f"new chain tip on main interface. {height=}")
             if blockchain_updated:
                 util.trigger_callback('blockchain_updated')
             util.trigger_callback('network_updated')