restore minimum lag for server swiching

lib/blockchain.py

@@ -94,7 +94,7 @@ class Blockchain(threading.Thread):
                 self.network.trigger_callback('updated')
 
             h = self.servers_height.get(self.network.interface.server)
-            if h is not None and h < height :
+            if h is not None and h < height - 1:
                 print_error( "Server is lagging", height, h)
                 if self.config.get('auto_cycle'):
                     self.network.set_server(i.server)