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/storage/databases/main/purge_events.py

@@ -42,17 +42,17 @@ class PurgeEventsStore(StateGroupWorkerStore, SQLBaseStore):
             The set of state groups that are referenced by deleted events.
         """
 
+        parsed_token = await RoomStreamToken.parse(self, token)
+
         return await self.db_pool.runInteraction(
             "purge_history",
             self._purge_history_txn,
             room_id,
-            token,
+            parsed_token,
             delete_local_events,
         )
 
-    def _purge_history_txn(self, txn, room_id, token_str, delete_local_events):
-        token = RoomStreamToken.parse(token_str)
-
+    def _purge_history_txn(self, txn, room_id, token, delete_local_events):
         # Tables that should be pruned:
         #     event_auth
         #     event_backward_extremities