Add memory pool based fee estimates

- fee estimates can use ETA or mempool
 - require protocol version 1.2
 - remove fee_unit preference

gui/kivy/uix/dialogs/settings.py

@@ -204,10 +204,7 @@ class SettingsDialog(Factory.Popup):
         d.open()
 
     def fee_status(self):
-        if self.config.get('dynamic_fees', True):
-            return fee_levels[self.config.get('fee_level', 2)]
-        else:
-            return self.app.format_amount_and_units(self.config.fee_per_kb()) + '/kB'
+        return self.config.get_fee_status()
 
     def fee_dialog(self, label, dt):
         if self._fee_dialog is None: