Fix stored messages growing infinitely on reloads

lib/tfw/networking/server/tfw_server.py

@@ -47,6 +47,7 @@ class TFWServer:
     def handle_recover(self, message):
         if message['key'] == 'recover':
             self._frontend_messages.replay_messages(self._uplink_connector)
+            self._frontend_messages.clear()
 
     def save_frontend_messages(self, message):
         self._frontend_messages.save_message(message)
@@ -70,6 +71,9 @@ class MessageStorage(ABC):
     def transform_message(self, message):   # pylint: disable=no-self-use
         yield message
 
+    def clear(self):
+        self.saved_messages.clear()
+
 
 class FrontendMessageStorage(MessageStorage):
     def filter_message(self, message):