Make token serializing/deserializing async (#8427)

The idea is that in future tokens will encode a mapping of instance to position. However, we don't want to include the full instance name in the string representation, so instead we'll have a mapping between instance name and an immutable integer ID in the DB that we can use instead. We'll then do the lookup when we serialize/deserialize the token (we could alternatively pass around an `Instance` type that includes both the name and ID, but that turns out to be a lot more invasive).

synapse/rest/client/v1/initial_sync.py

@@ -27,11 +27,12 @@ class InitialSyncRestServlet(RestServlet):
         super().__init__()
         self.initial_sync_handler = hs.get_initial_sync_handler()
         self.auth = hs.get_auth()
+        self.store = hs.get_datastore()
 
     async def on_GET(self, request):
         requester = await self.auth.get_user_by_req(request)
         as_client_event = b"raw" not in request.args
-        pagination_config = PaginationConfig.from_request(request)
+        pagination_config = await PaginationConfig.from_request(self.store, request)
         include_archived = parse_boolean(request, "archived", default=False)
         content = await self.initial_sync_handler.snapshot_all_rooms(
             user_id=requester.user.to_string(),