network: auto-switch servers to preferred fork (or longest chain)

If auto_connect is enabled, allow jumping between forks too.
(Previously auto_connect was only switching servers on a given fork,
not across forks)
If there is a preferred fork set, jump to that (and stay);
if there isn't, always jump to the longest fork.

electrum/interface.py

@@ -384,6 +384,7 @@ class Interface(PrintError):
             self.mark_ready()
             await self._process_header_at_tip()
             self.network.trigger_callback('network_updated')
+            await self.network.switch_unwanted_fork_interface()
             await self.network.switch_lagging_interface()
 
     async def _process_header_at_tip(self):