Fix StreamIdGenerator not handling unpersisted IDs

Spotted by @erikjohnston.

Closes #14456.

Author: erikjohnston

synapse/storage/util/id_generators.py

@@ -253,7 +253,7 @@ def manager() -> Generator[Sequence[int], None, None]:
         return _AsyncCtxManagerWrapper(manager())
 
     def get_current_token(self) -> int:
-        if self._is_writer:
+        if not self._is_writer:
             return self._current
 
         with self._lock: