Use nio's new restore_login() method

TODO.md

@@ -176,8 +176,6 @@
 
 ## Backend
 
-- Use new nio `AsyncClient.restore_login()`
-
 - Better config file format
 
 - Prevent starting multiple client instances, causes problems with E2E DB

src/backend/backend.py

@@ -233,8 +233,7 @@ class Backend:
         """Create and register a `MatrixClient` with known account details."""
 
         client = MatrixClient(
-            backend=self,
-            user=user_id, homeserver=homeserver, device_id=device_id,
+            self, user=user_id, homeserver=homeserver, device_id=device_id,
         )
 
         self.clients[user_id] = client

src/backend/matrix_client.py

@@ -293,18 +293,18 @@ class MatrixClient(nio.AsyncClient):
     async def resume(
         self,
         user_id:      str,
-        token:      str,
+        access_token: str,
         device_id:    str,
         state:        str = "online",
         status_msg:   str = "",
     ) -> None:
-        """Login to the server using an existing access token."""
+        """Restore a previous login to the server with a saved access token."""
+
+        self.restore_login(user_id, device_id, access_token)
 
-        response = nio.LoginResponse(user_id, device_id, token)
         account        = self.models["accounts"][user_id]
-        await self.receive_response(response)
-
         self._presence = "offline" if state == "invisible" else state
+
         account.set_fields(
             presence=Presence.State(state), status_msg=status_msg,
         )